Strong former US soldier taking on tough guys (8)
I believe the answer is:
vehement
'strong' is the definition.
(vehement can mean passionate or forceful)
'former us soldier taking on tough guys' is the wordplay.
'former us soldier' becomes 'vet' (US term for an army veteran).
'taking on' is an insertion indicator.
'tough guys' becomes 'hemen' (I've seen this in other clues).
'vet' going around 'hemen' is 'VEHEMENT'.
(Other definitions for vehement that I've seen before include "Showing strong feeling, like even them" , "Vigorously emphatic" , "Showing strong feeling, intense in speech" , "Intensely forceful" , "Forcible" .)