Found a ring was missing from the collection (4)
I believe the answer is:
fund
'collection' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for fund that I've seen before include "Pool of money" , "Provide with money, say project" , "Pool - float" , "Monetary resources" , "Finance" .)