Every ache is treated (4)
I believe the answer is:
each
'every' is the definition.
(synonyms)
'ache is treated' is the wordplay.
'is treated' indicates anagramming the letters.
'ache' with letters rearranged gives 'EACH'.
(Other definitions for each that I've seen before include "...per head" , "Singly, per capita" , "Every one separately" , "Every - apiece" , "every individual" .)