Every ache treated (4)
I believe the answer is:
each
'every' is the definition.
(synonyms)
'ache treated' is the wordplay.
'treated' indicates anagramming the letters.
'ache' anagrammed gives 'EACH'.
(Other definitions for each that I've seen before include "Every one separately" , "Every one of a number" , "This way for a place" , "Per capita, apiece" , "Ache (anag.)" .)