Simple bit of costume, really (4)

I believe the answer is:

mere

'simple' is the definition.
(I know that simple can be written as mere)

'bit of costume really' is the wordplay.
'bit of' says the answer is hidden in the clue.
'MERE' is hidden within 'costume really'.

(Other definitions for mere that I've seen before include "Pool -- pure and simple" , "Simply, nothing more than" , "DDwater" , "See 13" , "Pierre's mother identified" .)

I've seen this clue in The Irish News.

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