In the classics, he's out to be deviously cruel (8)
I believe the answer is:
hercules
'in the classics he's out to be deviously cruel' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he's out to be deviously cruel' is the wordplay.
'out' is an insertion indicator.
'to be deviously' indicates anagramming the letters (I've seen this in other clues).
'cruel' is an anagram of 'rcule'.
'hes' enclosing 'rcule' is 'HERCULES'.
Can you help me to learn more?
(Other definitions for hercules that I've seen before include "Mighty hero of Greek mythology" , "Mythical strongman celebrated for his "labours"" , "Greek hero who performed 12 labours to achieve immortality" , "Twelve-labours myth. hero" , "Labouring man" .)