Reassembling his gun - a pain! (7)
I believe the answer is:
anguish
'pain' is the definition.
('anguish' can be a synonym of 'pain')
'reassembling his gun a' is the wordplay.
'reassembling' is an anagram indicator.
'his'+'gun'+'a'='hisguna'
'hisguna' is an anagram of 'ANGUISH'.
(Other definitions for anguish that I've seen before include "Severe mental suffering" , "Severe pain, may be mental" , "extreme pain" , "Extreme misery" , "Extreme mental distress" .)