Bad month — need improving (2,3,4)
I believe the answer is:
on the mend
'improving' is the definition.
(I've seen this before)
'bad month need' is the wordplay.
'bad' is an anagram indicator.
'month'+'need'='monthneed'
'monthneed' anagrammed gives 'ON THE MEND'.
(Other definitions for on the mend that I've seen before include "Improving in health" , "Informally getting better, improving after illness or accident" , "convalescing" , "Looking up" , "undergoing recovery?" .)