Unopened kit characterises reserves (8)
I believe the answer is:
earmarks
'reserves' is the definition.
('earmark' can be a synonym of 'reserve')
'unopened kit characterises' is the wordplay.
'unopened' means to remove the first letter (without opening letter).
'kit' becomes 'gear' (I've seen this before).
'characterises' becomes 'marks' (marking is a kind of characterizing).
'gear' with its initial letter removed is 'ear'.
'ear'+'marks'='EARMARKS'
(Other definitions for earmarks that I've seen before include "Identifies" , "Sets aside, reserves" , "Allocates; reserves" , "Designates for a particular purpose" , "Puts distinguishing sign on - begins aurally" .)