Improving where further progress seemingly impossible, minutes saved (2,3,4)
I believe the answer is:
on the mend
'improving' is the definition.
(on the mend means improving after an illness)
'where further progress seemingly impossible minutes saved' is the wordplay.
'where further progress seemingly impossible' becomes 'on the end' (one cannot progress past the end).
'minutes' becomes 'm' (common abbreviation).
'saved' indicates putting letters inside (save can mean to keep hold of).
'ontheend' placed around 'm' is 'ON THE MEND'.
(Other definitions for on the mend that I've seen before include "Improving in health" , "returning to health" , "Making recovery" , "undergoing recovery?" , "convalescing" .)