Prevail over heartless monster (4)
I believe the answer is:
best
'prevail over' is the definition.
(thesaurus)
'heartless monster' is the wordplay.
'heartless' means to remove the middle letters.
'monster' becomes 'beast' (I've seen this before).
'beast' with its middle taken out is 'BEST'.
(Other definitions for best that I've seen before include "Top, finest" , "Superlative" , "Supreme" , "most fitting" , "Most outstanding" .)