The last bit following on them is improving (2,3,4)
I believe the answer is:
on the mend
'is improving' is the definition.
(on the mend means improving after an illness)
'the last bit following on them' is the wordplay.
'the last bit' becomes 'end' (I've seen this in another clue).
'following' says to put letters next to each other.
'on'+'them'='onthem'
'end' after 'onthem' is 'ON THE MEND'.
(Other definitions for on the mend that I've seen before include "Regaining health" , "convalescing" , "Looking up" , "Recovering from an injury or illness" , "returning to health" .)