Sit for a portrait (4)
I believe the answer is:
pose
'sit for a portrait' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pose that I've seen before include "Sit for picture" , "Attitudinise" , "Present (problem), sit" , "Present (a question); sit" , "Strike an attitude as for photo" .)