It's useless shielding a bad crook (7)
I believe the answer is:
villain
'crook' is the definition.
(thesaurus)
'it's useless shielding a bad' is the wordplay.
'it's useless' becomes 'vain' (I've seen this in another clue).
'shielding' is an insertion indicator.
'a bad' becomes 'ill' (I've seen this in another clue).
'vain' enclosing 'ill' is 'VILLAIN'.
(Other definitions for villain that I've seen before include "He may rob you" , "Bad person in story/play" , "Bad guy" , "Principal fictional baddie" , "Principal wicked character" .)