Picked up merchant's stock of wine (6)
I believe the answer is:
cellar
'stock of wine' is the definition.
(where wine is stored)
'picked up merchant's' is the wordplay.
'picked up' shows a homophone (sound like).
'merchant' becomes 'seller' (seller is a kind of merchant).
'seller' sounds like 'CELLAR'.
(Other definitions for cellar that I've seen before include "Room below ground for storage" , "Storage space, for wine perhaps" , "Underground area" , "Room where wine is stored (6)," , "Lower room" .)