He's cruel to upset the labourer (8)
I believe the answer is:
hercules
'the labourer' is the definition.
(I've seen this before)
'he's cruel to upset' is the wordplay.
'to upset' indicates an anagram.
'hes'+'cruel'='hescruel'
'hescruel' is an anagram of 'HERCULES'.
(Other definitions for hercules that I've seen before include "A very powerful man" , "Strong man of mythology" , "Mythical strongman celebrated for his "labours"" , "In Greek myth, a hero noted for great strength and 12 labours" , "Hero of Greek myth who performed 12 great labours" .)