Not in suitable clothes (6)
I believe the answer is:
outfit
'clothes' is the definition.
(outfit is a kind of clothes)
'not in suitable' is the wordplay.
'not in' becomes 'out' ('out' is the opposite to 'in').
'suitable' becomes 'fit' (similar in meaning).
'out'+'fit'='OUTFIT'
(Other definitions for outfit that I've seen before include "what to wear?" , "Set of clothes or group of people" , "Business" , "Ensemble of clothes" , "gear" .)