EXCLUSIVE

Steve Nash's attorney is grilling the NBA star's ex-wife like a cheeseburger ... accusing Alejandra of scheming behind Steve's back so she could trap him into paying child support, TMZ has learned.

Alejandra Amarilla is currently on the stand in an Arizona courtroom in the former couple's nasty custody hearing ... and things are getting pretty intense.

Moments ago, Steve's lawyer hammered Alejandra in open court -- saying, "You saw the trade [from the Phoenix Suns to the Lakers] as a way to obtain a higher child support amount regardless of what was best for the kids, didn't you?"

Alejandra denied the allegation -- shooting back with a simple, "No."

Then, Nash's attorney produced an email that Alejandra supposedly sent to a 3rd party -- in which it appears Alejandra discusses a plot to trap Nash into paying child support.

The email reads -- "My position is we are moving to California because [our children's] father is there. I believe the school is a great fit for them. I can't buy a home unless we get child support. Plan is to move to CA. Wait 6 months then get court involved in child support."

As we previously reported, Alejandra is adamant she only wants to move to L.A. so the kids can spend more time with their father -- who plays for the Lakers and lives in Manhattan Beach.