Ban Doctor Who at last (4)
I believe the answer is:
veto
'ban' is the definition.
(thesaurus)
'doctor who at last' is the wordplay.
'doctor' becomes 'vet' (vet is a kind of doctor).
'at last' says to take the final letters.
The last letter of 'who' is 'o'.
'vet'+'o'='VETO'
(Other definitions for veto that I've seen before include "Refusal to assent" , "Vote that blocks a decision" , "Throw out" , "An overriding negative vote" , "Reject - ban" .)