WITH Lachie Henderson now almost certainly headed to Geelong and Robbie Tarrant re-signed at North Melbourne, Essendon's Jake Carlisle is now indisputably the hottest property in a market desperate for key defenders.

The Brisbane Lions, Western Bulldogs, Geelong, Carlton, North Melbourne, Fremantle, St Kilda and Hawthorn are among the clubs believed to be actively in the market for a key defender this year.

And but for their AFL-imposed trade restriction, the Sydney Swans would be jostling right alongside them.

At the start of 2015, these clubs had a reasonably deep pool of free agents and uncontracted players to choose from.

But that pool has since become shallower than the plot of a Police Academy movie, with Carlisle the only 'gettable' A-grade defender remaining.

With Tarrant re-signing with North Melbourne last Friday, there are no long-term prospects left among this year's free agents.

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Michael Johnson, Alex Rance and Heath Grundy recommitted to their respective clubs some time ago, while Dustin Fletcher will almost certainly retire, and Luke McPharlin and Ted Richards will either follow him into the football after-life or play on at their existing clubs.

At 191cm, restricted free agent Colin Garland is more a third tall defender, but it's believed he and Melbourne remain some distance apart in contract negotiations, so clubs looking to bolster their backlines could yet make serious plays for him.

The out-of-contract market is only marginally healthier.

Henderson is technically available after informing Carlton on Thursday that he wants a trade, but the former Geelong Falcon is believed to have his heart set on joining the Cats and given Geelong's record of orchestrating win-win trades we expect that deal will get done.

Which means every other club now has to look elsewhere.

Gold Coast's Rory Thompson has attracted significant interest but, while his contract talks with the Suns are dragging on, rival clubs are resigned to the fact he'll remain at Metricon Stadium beyond 2015.

Sun Rory Thompson is expected to remain on the Gold Coast. Picture: AFL Media

Clubs could consider uncontracted Hawk Ryan Schoenmakers, but if they're after a key defender with star qualities Carlisle is their only remaining option.

Like Henderson, Carlisle has spent time in attack in recent seasons but played his most consistent football in the backline.

Clubs are prepared to offer him up to $700,000 to bolster their defences from 2016.

The pack leading the chase for Carlisle is believed to include Carlton, North Melbourne and the Western Bulldogs.

The Bomber would also be a perfect fit for the Lions and Fremantle, but he is unlikely to leave Victoria.

Carlisle is believed to have been involved in Essendon's 2011-12 supplements program and faces a possible playing ban if the World Anti-Doping Agency (WADA) is successful in its appeal to the Court of Arbitration for Sport later this year.

But this has not diminished his appeal to rival suitors.

Not when he's still 23. And especially not when he's the best available key defender in a market starved of that species.

It follows then that if his teammate and fellow key defender Cale Hooker enters 2016 out of contract, he will be just as hot a property, most likely even hotter.

Hooker headlines the key defenders set to become free agents at the end of next season, with Collingwood's Nathan Brown and West Coast pair Mitch Brown and Will Schofield also potentially in that pool.

All four could discover next year – as Carlisle has in 2015 – that there has never been a better time to be an out-of-contract key defender.