Treatment for ache in a head (4)
I believe the answer is:
each
'a head' is the definition.
(I've seen this before)
'treatment for ache' is the wordplay.
'treatment for' indicates an anagram (I've seen this in other clues).
'ache' with letters rearranged gives 'EACH'.
'in' is the link.
(Other definitions for each that I've seen before include "Every single one, person or thing" , "every one of a group" , "Per capita, apiece" , "Every - apiece" , "Per person" .)