Stock of wine offered by merchant on the radio (6)
I believe the answer is:
cellar
'stock of wine' is the definition.
(where wine is stored)
'merchant on the radio' is the wordplay.
'merchant' becomes 'seller' (seller is a kind of merchant).
'on the radio' shows a homophone (sound like).
'seller' sounds like 'CELLAR'.
'offered by' acts as a link.
(Other definitions for cellar that I've seen before include "Storage space below ground" , "Lower room" , "Where provisions may be stored" , "Wine stock; sounds like vendor" , "Underground storeroom" .)