Ban doctor beginning to operate (4)
I believe the answer is:
veto
'ban' is the definition.
(veto can mean a ban or prohibition on something)
'doctor beginning to operate' is the wordplay.
'doctor' becomes 'vet' (vetting is a kind of doctoring).
'beginning to' suggests taking the first letters.
The first letter of 'operate' is 'o'.
'vet'+'o'='VETO'
(Other definitions for veto that I've seen before include "Right to prevent proposed action" , "Authoritative prohibition" , "Embargo" , "Right to reject a proposal" , "The power to prohibit or reject a proposal" .)