Apt rhyme for "grab" (3)
I believe the answer is:
'apt rhyme for grab' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nab that I've seen before include "Catch (thief) (colloq.)" , "Capture (slang)" , "Take" , "Catch; grab suddenly" , "Apprehend" .)