Not in appropriate costume (6)
I believe the answer is:
outfit
'costume' is the definition.
(thesaurus)
'not in appropriate' is the wordplay.
'not in' becomes 'out' ('out' is the opposite to 'in').
'appropriate' becomes 'fit' ('fit' can be similar in meaning to 'appropriate').
'out'+'fit'='OUTFIT'
(Other definitions for outfit that I've seen before include "suit" , "Set of clothes or group of people" , "Complete set of clothes" , "Organistion or set of clothing" , "Costume" .)