Wicked discount for dons (9)
I believe the answer is:
reprobate
'wicked' is the definition.
(thesaurus)
'discount for dons' is the wordplay.
'discount' becomes 'rebate' (synonyms).
'for' becomes 'pro' (as in being 'pro' something).
'dons' means one lot of letters goes inside another (inserted letters wear or \'don\' others).
'rebate' placed around 'pro' is 'REPROBATE'.
(Other definitions for reprobate that I've seen before include "Rascal" , "immoral" , "Profligate" , "someone depraved" , "corrupt" .)