Right about costume (7)
I believe the answer is:
redress
'right' is the definition.
('redress' can be a synonym of 'right')
'about costume' is the wordplay.
'about' becomes 're' (regarding).
'costume' becomes 'dress' (I've seen this before).
're'+'dress'='REDRESS'
(Other definitions for redress that I've seen before include "Set right - compensation" , "Get back into gear" , "relief" , "Prepare again - satisfaction" , "remedy" .)