Need month to improve, getting better (2,3,4)
I believe the answer is:
on the mend
'getting better' is the definition.
(on the mend means recovering from illness)
'need month to improve' is the wordplay.
'to improve' is an anagram indicator.
'need'+'month'='needmonth'
'needmonth' with letters rearranged gives 'ON THE MEND'.
(Other definitions for on the mend that I've seen before include "returning to health" , "undergoing recovery?" , "convalescing" , "Making recovery" , "Becoming better in health" .)