Mac's breeding a bastard? (9)
I believe the answer is:
gaberdine
'mac's' is the definition.
(I've seen this before)
'breeding a bastard?' is the wordplay.
'bastard?' indicates an anagram (bastard can mean impure or mixed).
'breeding'+'a'='breedinga'
'breedinga' anagrammed gives 'GABERDINE'.
(Other definitions for gaberdine that I've seen before include "Mac; began ride (anag.)" , "twill" , "Raincoat fabric" .)