Longs to arrest evil hero (8)
I believe the answer is:
achilles
'hero' is the definition.
(I've seen this before)
'longs to arrest evil' is the wordplay.
'longs' becomes 'aches' ('ache' can be a synonym of 'long').
'to arrest' means one lot of letters goes inside another (arrest can mean capture).
'evil' becomes 'ill' (as in doing people ill).
'aches' enclosing 'ill' is 'ACHILLES'.
(Other definitions for achilles that I've seen before include "Ancient soldier" , "Mythical hero, vulnerable only through his heel" , "Old fighter" , "Legendary Greek with bad heel" , "Homeric hero" .)