Unavailing, nursing sick criminal (7)
I believe the answer is:
villain
'criminal' is the definition.
(thesaurus)
'unavailing nursing sick' is the wordplay.
'unavailing' becomes 'vain'.
'nursing' means one lot of letters goes inside another.
'sick' becomes 'ill' ('ill' can be a synonym of 'sick').
'vain' enclosing 'ill' is 'VILLAIN'.
(Other definitions for villain that I've seen before include "Scoundrel" , "Bad person in story/play" , "He may rob you" , "Principal fictional baddie" , "Wrongdoer" .)