Two-piece bathing costume (6)
I believe the answer is:
bikini
'two-piece bathing costume' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bikini that I've seen before include "gear in two pieces" , "tests were carried out here" , "Women's tiny bathing costume" , "Atoll and skimpy swimsuit" , "Small two-piece bathing costume" .)