Condiment that's assembled, by the sound of it (7)
I believe the answer is:
mustard
'condiment' is the definition.
(I know that mustard is a type of condiment)
'assembled by the sound of it' is the wordplay.
'assembled' becomes 'mustered' (muster means to gather).
'by the sound of it' shows a homophone (sound like).
'mustered' is a homophone of 'MUSTARD'.
'that's' acts as a link.
(Other definitions for mustard that I've seen before include "The keen stuff ?" , "Type of Brassica" , "Hot stuff!" , "Hot-tasting seed" , "Hot-tasting condiment" .)