After heading off gun injury, relaxed (7)
I believe the answer is:
unwound
'relaxed' is the definition.
('unwind' can be a synonym of 'relax')
'after heading off gun injury' is the wordplay.
'after' means one lot of letters go next to another.
'heading off' means to remove the first letter.
'injury' becomes 'wound' (wound is a kind of injury**).
'gun' with its initial letter removed is 'un'.
'un'+'wound' is 'UNWOUND'.
(Other definitions for unwound that I've seen before include "Bandages thus removed" , "No longer coiled" , "Slackened, relaxed" , "Unravelled" , "Began taking things easy" .)