Fixed up mate with stock option (3)
I believe the answer is:
put
'stock option' is the definition.
(I've seen this in another clue)
'fixed up mate' is the wordplay.
'fixed up' says the letters should be written backwards (in a down clue, letters go up).
'mate' becomes 'tup' (both can mean to copulate).
'tup' backwards is 'PUT'.
'with' acts as a link.
(Other definitions for put that I've seen before include "Place in a position" , "Set (in place)" , "Place, lay" , "Place in a location" , "Place; placed" .)