An opening for doctor to accept new start (4)
I believe the answer is:
vent
'an opening' is the definition.
(vent is a kind of opening)
'doctor to accept new start' is the wordplay.
'doctor' becomes 'vet' (vet is a kind of doctor**).
'to accept' means one lot of letters goes inside another.
'start' suggests taking the first letters.
The first letter of 'new' is 'n'.
'vet' placed around 'n' is 'VENT'.
'for' acts as a link.
(Other definitions for vent that I've seen before include "Voice" , "pipe" , "Orifice" , "Opening for air" , "Opening releasing gas" .)