Alloy, bargain by the sound of it? (5)
I believe the answer is:
steel
'alloy' is the definition.
(steel is a kind of alloy)
'bargain by the sound of it?' is the wordplay.
'bargain' becomes 'steal' (synonyms).
'by the sound of it?' shows a homophone (sound like).
'steal' is a homophone of 'STEEL'.
(Other definitions for steel that I've seen before include "Iron/carbon alloy" , "Form of iron" , "brace" , "Mixture of carbon and iron" , "Make hard and unfeeling" .)