Capricious fellow concealing deserter (7)
I believe the answer is:
erratic
'capricious' is the definition.
(thesaurus)
'fellow concealing deserter' is the wordplay.
'fellow' becomes 'eric' (I've seen this in other clues**).
'concealing' indicates putting letters inside.
'deserter' becomes 'rat' (I've seen this before).
'eric' enclosing 'rat' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Not even" , "on irregular path" , "Variable" , "Rambling" , "Irregular, capricious" .)