Active soldier, bitter outside (5)
I believe the answer is:
agile
'active' is the definition.
(I've seen this before)
'soldier bitter outside' is the wordplay.
'soldier' becomes 'gi' (government issue or general issue).
'bitter' becomes 'ale' ('ale' can be a synonym of 'bitter').
'outside' is an insertion indicator.
'gi' inserted into 'ale' is 'AGILE'.
(Other definitions for agile that I've seen before include "Moving quickly/easily" , "Flexible" , "Nimble, supple" , "athletic" , "Able to move nimbly" .)