Treated allergy, to a great extent (7)
I believe the answer is:
largely
'to a great extent' is the definition.
(I've seen this before)
'treated allergy' is the wordplay.
'treated' indicates an anagram.
'allergy' is an anagram of 'LARGELY'.
(Other definitions for largely that I've seen before include "to a considerable extent" , "On the whole" , "To a great extent" , "In the main" , "as a rule" .)