Bikini for one adult comes with cost (5)
I believe the answer is:
atoll
'bikini' is the definition.
(Bikini Atoll is an example)
'one adult comes with cost' is the wordplay.
'one adult' becomes 'a' (abbreviation).
'comes with' means one lot of letters go next to another.
'cost' becomes 'toll' (toll is a kind of cost).
'a'+'toll'='ATOLL'
'for' acts as a link.
(Other definitions for atoll that I've seen before include "Coral circle" , "Coral isle" , "tropical paradise?" , "Bikini for one" , "Pacific coral island" .)