Perplexed to get the sack when full of inspiration (7)
I believe the answer is:
bemused
'perplexed' is the definition.
(similar in meaning)
'the sack when full of inspiration' is the wordplay.
'the sack' becomes 'bed' ('in the sack' means 'in bed').
'when full of' means one lot of letters goes inside another.
'inspiration' becomes 'muse'.
'bed' going around 'muse' is 'BEMUSED'.
'to get' is the link.
(Other definitions for bemused that I've seen before include "Puzzled" , "Deep in thought" , "Dazed" , "In the course of puzzling" , "Perplexed, bewildered" .)