Strongman upsets masons (6)
I believe the answer is:
samson
'strongman' is the definition.
(I have seen 'Biblical strongman' mean 'samson' so perhaps 'strongman' could also mean 'samson')
'upsets masons' is the wordplay.
'upsets' is an anagram indicator.
'masons' anagrammed gives 'SAMSON'.
(Other definitions for samson that I've seen before include "Biblical strong man who got haircut" , "Israelite leader famous for his strength" , "Old Testament strongman (until he lost his hair)" , "Delilah's muscular boy friend" , "Delilah sheared him" .)