In favour of stopping discount for ne'er-do-well (9)
I believe the answer is:
reprobate
'ne'er-do-well' is the definition.
(both can mean a good-for-nothing person)
'in favour of stopping discount' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'stopping' indicates putting letters inside (stopping can mean blocking a gap).
'discount' becomes 'rebate' (synonyms).
'pro' placed within 'rebate' is 'REPROBATE'.
'for' acts as a link.
(Other definitions for reprobate that I've seen before include "Sinner" , "scoundrel" , "immoral" , "17 [WRONGDOER]" , "Rogue" .)