Lacking any definite scale for a beneficial medicine (6)
I believe the answer is:
atonic
'lacking any definite scale for a beneficial medicine' is the definition.
I can't tell whether this definition defines the answer.
'a beneficial medicine' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'medicine' could be 'tonic' (tonic is a kind of medicine) and 'tonic' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for atonic that I've seen before include "(In poetry) unaccented" , "Unstressed (poetry)" .)