Molluscs for Roy's set, possibly (7)
I believe the answer is:
oysters
'molluscs' is the definition.
(oyster is a kind of mollusc)
'roy's set possibly' is the wordplay.
'possibly' indicates anagramming the letters.
'roys'+'set'='roysset'
'roysset' anagrammed gives 'OYSTERS'.
'for' acts as a link.
(Other definitions for oysters that I've seen before include "Yet Ross, strangely, has the marine creatures" , "Yet Ross can find the marine creatures" , "Succulent bivales" , "Sources of pearls" , "Yet Ross has these strange marine creatures" .)