Single fee for the main feature (5)
I believe the answer is:
atoll
'the main feature' is the definition.
(reef in the sea - 'the main' is a name for the sea)
'single fee' is the wordplay.
'single' becomes 'a'.
'fee' becomes 'toll' (toll is a kind of fee).
'a'+'toll'='ATOLL'
'for' acts as a link.
(Other definitions for atoll that I've seen before include "Ring of coral" , "land threatened by rising sea levels" , "tropical paradise?" , "Bikini, perhaps" , "Land around a lagoon, such as Bikini" .)