Fix setter's getting sack (5)
I believe the answer is:
imbed
'fix' is the definition.
(thesaurus)
'setter's getting sack' is the wordplay.
'setter's' becomes 'im' (I've seen this in other clues).
'getting' says to put letters next to each other.
'sack' becomes 'bed' ('in the sack' means 'in bed').
'im'+'bed'='IMBED'
(Other definitions for imbed that I've seen before include "Fix firmly and deeply" , "Become deeply fixed" , "Plant" , "Implant" , "Fix deeply into surroundings" .)