ESSENDON is closing in on a back-up for No.1 ruckman Tom Bellchambers, with former Docker Zac Clarke planning to sign under new AFL rookie rules.

Both parties are waiting on the AFL to give the arrangement the green light ahead of Friday's second list lodgement deadline, because there is still confusion around how the rule works.

It could be the same situation for fellow ruckman Shane Mumford, who is set to come out of retirement for Greater Western Sydney but wants to avoid the draft.

Neither Clarke nor Mumford qualifies as a delisted free agent because they've each had a season out of the AFL.

However, as players who were previously on an AFL list they could sign as Category A rookies between December 1 and March 15 if League headquarters gives approval.

That will depend on how the AFL officiates the rule, given it was originally reported as being introduced to help clubs cover a footballer who retires or suffers a serious, long-term injury.

Clarke, who played 101 games and kicked 46 goals for Fremantle, will have to nominate for the draft if AFL decision-makers deem him ineligible to sign that way.

The 203cm giant underwent a medical at a different club to the Bombers and was set for another one with a third club this week, so Essendon would face competition in a draft scenario.

The Dockers cut Clarke at the end of the 2017 season after two surgeries and related complications with the patella in his left knee ruined his last year there.

But the 28-year-old bounced back this year to star with eventual WAFL premier Subiaco and would fill the void at the Bombers left by the surprise retirement of Matthew Leuenberger.

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The Leuenberger situation may help Essendon's case, with his decision to call it quits – despite having a contract in front of him – leaving it with only 20-year-old ruckman Sam Draper behind Bellchambers.

Clarke made 20 WAFL appearances and kicked 28 goals in the 2018 season, while averaging 15.7 disposals, 4.6 marks and 39 hit-outs.

He had at least 50 hit-outs in his last three matches this past season, including the Lions' Grand Final triumph over West Perth, and managed the feat twice more before that.

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The Bombers also conducted a medical on delisted Western Bulldogs big man Tom Campbell, but his search for a fresh AFL start looks set to continue.

There are other options for Campbell, with North Melbourne, Richmond, the Western Bulldogs and Adelaide among the clubs potentially on the hunt for an experienced back-up.

Helping the 42-game former Dog's cause is the thin ruck depth in this year's draft market, with a little-known 205cm rugby convert, Jack Blair, surging into calculations.

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Blair, 23, is training with Sydney and AFL.com.au understands at least one Melbourne club has held discussions with his management.