This is to constrain oddly cruel strongman (8)
I believe the answer is:
hercules
'strongman' is the definition.
'this is to constrain oddly cruel' is the wordplay.
'this is' becomes 'here's' (both can be used to point out someone or something).
'to constrain' indicates putting letters inside (constrain can mean to imprison).
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
The alternating letters of 'cruel' are 'cul'.
'heres' enclosing 'cul' is 'HERCULES'.
(Other definitions for hercules that I've seen before include "Mythological hero" , "labourer" , "Labouring man" , "Twelve-labours myth. hero" , "In Greek myth, a hero noted for great strength and 12 labours" .)