Blackguard being bad in vain (7)
I believe the answer is:
villain
'blackguard' is the definition.
(thesaurus)
'bad in vain' is the wordplay.
'bad' becomes 'ill' (similar in meaning).
'in' means one lot of letters goes inside another.
'ill' inserted inside 'vain' is 'VILLAIN'.
'being' acts as a link.
(Other definitions for villain that I've seen before include "One played badly" , "Criminal, rogue" , "Scoundrel" , "Evil character" , "He may rob you" .)