Recovering, continuously injecting them (2,3,4)
I believe the answer is:
on the mend
'recovering' is the definition.
(I've seen this before)
'continuously injecting them' is the wordplay.
'continuously' becomes 'onend' (I've seen this before).
'injecting' is an insertion indicator.
'onend' placed around 'them' is 'ON THE MEND'.
(Other definitions for on the mend that I've seen before include "Becoming better in health" , "returning to health" , "Recovering from an injury or illness" , "Improving in health" , "Making recovery" .)