Bikini Atoll's first insect returns (5)
I believe the answer is:
tanga
'bikini' is the definition.
(a Tanga bikini is a kind of bikini)
'atoll's first insect returns' is the wordplay.
'atoll's first' becomes 'a' (gst letter of 'atoll').
'insect' becomes 'gnat' (gnat is a kind of insect).
'returns' says the letters should be written in reverse.
'a'+'gnat'='agnat'
'agnat' reversed gives 'TANGA'.
(Other definitions for tanga that I've seen before include "Very brief bikini" , "Briefs of two small panels connected by strings" , "underwear" , "Briefs not covering hips" , "Very brief pants" .)