Recovering, continuously swallowing them (2,3,4)
I believe the answer is:
on the mend
'recovering' is the definition.
(on the mend means recovering or recuperating)
'continuously swallowing them' is the wordplay.
'continuously' becomes 'on end' (eg in a phrase like 'for weeks on end').
'swallowing' indicates putting letters inside.
'onend' enclosing 'them' is 'ON THE MEND'.
(Other definitions for on the mend that I've seen before include "Becoming better in health" , "Improving in health" , "convalescing" , "Recovering from an injury or illness" , "Informally getting better, improving after illness or accident" .)