Allergy treated, on the whole (7)
I believe the answer is:
largely
'on the whole' is the definition.
(I've seen this before)
'allergy treated' is the wordplay.
'treated' is an anagram indicator.
'allergy' with letters rearranged gives 'LARGELY'.
(Other definitions for largely that I've seen before include "Generally" , "To a great extent" , "Allergy (anag.)" , "to a considerable extent" , "In the main" .)