Puzzled, needing source of inspiration in the sack (7)
I believe the answer is:
bemused
'puzzled' is the definition.
(being bemused is being puzzled or confused)
'source of inspiration in the sack' is the wordplay.
'source of inspiration' becomes 'muse' (I've seen this in another clue).
'in' indicates putting letters inside.
'the sack' becomes 'bed' ('in the sack' means 'in bed').
'muse' going into 'bed' is 'BEMUSED'.
'needing' is the link.
(Other definitions for bemused that I've seen before include "In the course of puzzling" , "Floored" , "Dazed" , "Confused, bewildered" , "Stupefied" .)