Treated allergy, for the most part (7)
I believe the answer is:
largely
'for the most part' is the definition.
(synonyms)
'treated allergy' is the wordplay.
'treated' indicates anagramming the letters.
'allergy' with letters rearranged gives 'LARGELY'.
(Other definitions for largely that I've seen before include "Mostly found in gallery" , "Allergy (anag.)" , "Principally" , "To a great extent" , "Fro the most part" .)