Need month for treatment, improving (2,3,4)
I believe the answer is:
on the mend
'improving' is the definition.
(on the mend means improving after an illness)
'need month for treatment' is the wordplay.
'for treatment' indicates an anagram (the anagram must be processed or treated).
'need'+'month'='needmonth'
'needmonth' with letters rearranged gives 'ON THE MEND'.
(Other definitions for on the mend that I've seen before include "undergoing recovery?" , "Improving in health" , "Looking up" , "Recovering from an injury or illness" , "Making recovery" .)