Key from a ring? (5)
I believe the answer is:
atoll
'key' is the definition.
(thesaurus)
'a ring?' is the wordplay.
'ring?' becomes 'toll' (tolling is a kind of ringing).
'a'+'toll'='ATOLL'
'from' is the link.
(Other definitions for atoll that I've seen before include "String of islands" , "Bikini for one" , "Bikini, say" , "Ring-shaped coral reef enclosing a lagoon" , "Coral island with central lagoon" .)