Hard stuff that's a bargain, say (5)
I believe the answer is:
steel
'hard stuff' is the definition.
(I've seen this in another clue)
'a bargain say' is the wordplay.
'a bargain' becomes 'steal' (synonyms).
'say' shows a homophone (sound like).
'steal' is a homophone of 'STEEL'.
'that's' acts as a link.
(Other definitions for steel that I've seen before include "Structural metal alloy" , "Determination, metal" , "Form of iron" , "Strong metal" , "brace" .)