Puzzled birds in sack (7)
I believe the answer is:
bemused
'puzzled' is the definition.
(being bemused is being puzzled or confused)
'birds in sack' is the wordplay.
'birds' becomes 'emus' (emu is a kind of bird).
'in' indicates putting letters inside.
'sack' becomes 'bed' ('in the sack' means 'in bed').
'emus' inserted into 'bed' is 'BEMUSED'.
(Other definitions for bemused that I've seen before include "Dazed" , "In the course of puzzling" , "Perplexed or absorbed in thought" , "Floored" , "Confused, bewildered" .)