Single fragment of a modest swimsuit (3-5)
I believe the answer is:
one-piece
'a modest swimsuit' is the definition.
(I've seen this in another clue)
'single fragment' is the wordplay.
'single' becomes 'one' (similar in meaning).
'fragment' becomes 'piece' (I've seen this in another clue).
'one'+'piece'='ONE-PIECE'
'of' acts as a link.
(Other definitions for one-piece that I've seen before include "Made as a single item" , "Bathing-costume" , "Made as a single unit" , "(E.g. swimming costume) not as separates" .)