Heard vendor in vault (6)
I believe the answer is:
cellar
'vault' is the definition.
(I've seen this before)
'heard vendor' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'vendor' becomes 'seller' ('seller' can be a synonym of 'vendor').
'seller' is a homophone of 'CELLAR'.
'in' is the link.
(Other definitions for cellar that I've seen before include "Space for storing wine, say" , "Room where wine is stored (6)," , "Storage space below ground" , "Store for wine, possibly" , "Basement store" .)