Doctor's cures in hospital a lifesaver (7)
I believe the answer is:
rescuer
'a lifesaver' is the definition.
(thesaurus)
'doctor's cures in hospital' is the wordplay.
'doctor's' indicates anagramming the letters (I've seen 'doctors' mean this).
'in hospital' becomes 'er' (I've seen this in other clues**).
'cures' anagrammed gives 'rescu'.
'rescu'+'er'='RESCUER'
(Other definitions for rescuer that I've seen before include "freer" , "Deliverer (from danger)" , "Person bringing one out of danger" , "One who saves from a dangerous situation" , "One who saves someone" .)