Underground store for a vendor, so we hear (6)
I believe the answer is:
cellar
'underground store' is the definition.
(I've seen this before)
'vendor so we hear' is the wordplay.
'vendor' becomes 'seller' (synonyms).
'so we hear' indicates a 'sounds like' (homophone) clue.
'seller' sounds like 'CELLAR'.
'for a' is the link.
(Other definitions for cellar that I've seen before include "Storage area for the wine" , "It holds wine (or salt)" , "Bottom room where wine is often stored" , "Store for wine, possibly" , "Room where wine is stored (6)," .)