A trite form of costume (6)
I believe the answer is:
attire
'costume' is the definition.
(I've seen this in another clue)
'a trite form' is the wordplay.
'form' indicates an anagram (another form of the letters).
'trite' anagrammed gives 'ttire'.
'a'+'ttire'='ATTIRE'
'of' is the link.
(Other definitions for attire that I've seen before include "Clothing, apparel" , "Prepare" , "Habit" , "Formal clothing" , "Wear" .)