Mortise's mate (5)
I believe the answer is:
tenon
'mortise's mate' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tenon that I've seen before include "Piece to fit into mortise" , "Projection (joins with mortise)" , "Part of joint that fits in mortise" , "Mortise filler" , "Join" .)