Droop after shaking off head pain (7)
I believe the answer is:
anguish
'pain' is the definition.
('anguish' can be a synonym of 'pain')
'droop after shaking off head' is the wordplay.
'droop' becomes 'languish' (I've seen this before).
'after shaking off head' means to remove the first letter.
'languish' with its initial letter taken off is 'ANGUISH'.
(Other definitions for anguish that I've seen before include "Angst" , "torture" , "Severe pain, may be mental" , "Hi, Angus - that hurts" , "Severe mental suffering" .)