Capricious boy captures deserter (7)
I believe the answer is:
erratic
'capricious' is the definition.
(I've seen this before)
'boy captures deserter' is the wordplay.
'boy' becomes 'eric' (I've seen this before).
'captures' is an insertion indicator.
'deserter' becomes 'rat' (both can mean a traitor).
'eric' enclosing 'rat' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Wayward" , "Unpredictable, temperamental" , "Haphazard" , "Behaving irregularly" , "Uneven in movement or quality" .)