Lessening of dose perhaps cured it, on receiving treatment (9)
I believe the answer is:
reduction
'lessening of dose' is the definition.
(reducing the dose of medicine)
'cured it on receiving treatment' is the wordplay.
'receiving treatment' indicates an anagram (letters fixed or treated).
'cured'+'it'+'on'='curediton'
'curediton' anagrammed gives 'REDUCTION'.
'perhaps' is the link.
(Other definitions for reduction that I've seen before include "Introduce by way of discount" , "Introduce (anag) - decrease" , "Drop in price" , "Lessening" , "cutback!" .)