Plain clothes fit? Um, badly! (5)
I believe the answer is:
mufti
'plain clothes' is the definition.
(I've seen this before)
'fit? um badly' is the wordplay.
'badly' indicates an anagram.
'fit'+'um'='fitum'
'fitum' with letters rearranged gives 'MUFTI'.
(Other definitions for mufti that I've seen before include "Civilian attire" , "No official dress" , "For soldiers, civilian dress" , "Plain clothes worn by soldier" , "Ordinary clothes" .)