Merchandise, variable and heavily built (6)
I believe the answer is:
stocky
'heavily built' is the definition.
(I've seen this before)
'merchandise variable' is the wordplay.
'merchandise' becomes 'stock' (stock is a kind of merchandise).
'variable' becomes 'y' (algebra).
'stock'+'y'='STOCKY'
'and' acts as a link.
(Other definitions for stocky that I've seen before include "Heavily-built" , "Of short and solid stature" , "Of sturdy build" , "Broad" , "Strongly-built" .)