Hero outlaw guards cash machine (6)
I believe the answer is:
batman
'hero' is the definition.
(I know this)
'outlaw guards cash machine' is the wordplay.
'outlaw' becomes 'ban' (banning is a kind of outlawing).
'guards' indicates putting letters inside (inserted letters are guarded).
'cash machine' becomes 'a t m' (synonyms).
'ban' placed around 'atm' is 'BATMAN'.
(Other definitions for batman that I've seen before include "Comic/film superhero" , "who'd fight crime?" , "cartoon character" , "Tender" , "gives help to one in a mess?" .)