Certainly, doctor hit nurse with end of ring (4,5)
I believe the answer is:
sure thing
'certainly' is the definition.
(thesaurus)
'doctor hit nurse with end of ring' is the wordplay.
'doctor' indicates anagramming the letters.
'with' is a charade indicator (letters next to each other).
'end of' suggests the final letters.
The final letter of 'ring' is 'g'.
'hit'+'nurse'='hitnurse'
'hitnurse' anagrammed gives 'surethin'.
'surethin'+'g'='SURE THING'
(Other definitions for sure thing that I've seen before include "cert" , "of course" , "no chance of losing" , "A certainty - I agree!" .)