Two-piece swimming costume (6)
I believe the answer is:
bikini
'two-piece swimming costume' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bikini that I've seen before include "Small two-piece bathing suit" , "Infamous atoll" , "Atoll and swimming costume" , "Skimpy two-piece swimsuit" , "two-piece outfit" .)