Treat ache for every one (4)
I believe the answer is:
each
'every one' is the definition.
(thesaurus)
'treat ache' is the wordplay.
'treat' indicates an anagram.
'ache' with letters rearranged gives 'EACH'.
'for' is the link.
(Other definitions for each that I've seen before include "every one of a group" , "Per capita, apiece" , "Every - apiece" , "every individual" , "Per person" .)