Reserve heading off in pain (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'reserve heading off' is the wordplay.
'reserve' becomes 'cache' (I've seen this before).
'heading off' means to remove the first letter.
'cache' with its initial letter taken off is 'ACHE'.
'in' acts as a link.
(Other definitions for ache that I've seen before include "Dull prolonged pain" , "Cause pain" , "Pine" , "Similar" , "Hanker" .)