Alloy that's a bargain, by the sound of it (5)
I believe the answer is:
steel
'alloy' is the definition.
(I know that steel is a type of alloy)
'bargain by the sound of it' is the wordplay.
'bargain' becomes 'steal' (synonyms).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'steal' is a homophone of 'STEEL'.
'that's a' is the link.
(Other definitions for steel that I've seen before include "The staying power" , "nerve" , "Form of iron" , "May be cold" , "Make hard and unfeeling" .)