Most painful area itches terribly (7)
I believe the answer is:
achiest
'most painful' is the definition.
('achy' can be similar in meaning to 'painful')
'area itches terribly' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'terribly' is an anagram indicator.
'itches' with letters rearranged gives 'chiest'.
'a'+'chiest'='ACHIEST'
(Other definitions for achiest that I've seen before include "suffering most pain" , "most painful" .)