Revealing costume: nothing stops brother wearing one (8)
I believe the answer is:
monokini
'revealing costume' is the definition.
(type of bikini)
'nothing stops brother wearing one' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'stops' means one lot of letters goes inside another.
'brother' becomes 'monk' (I've seen this before).
'wearing one' becomes 'ini' (I can't explain this - if you can you should believe this answer much more).
'o' inserted inside 'monk' is 'monok'.
'monok'+'ini'='MONOKINI'
(Other definitions for monokini that I've seen before include "Skimpy garment" , "Swimsuit" , "Small beach garment" , "pair of nifty briefs?" .)