Put a block on Doctor Who at last (4)
I believe the answer is:
veto
'put a block' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'on doctor who at last' is the wordplay.
'on' says to put letters next to each other.
'doctor' becomes 'vet' (vet is a kind of doctor).
'at last' indicates one should take the final letters.
The last letter of 'who' is 'o'.
'vet' next to 'o' is 'VETO'.
Can you help me to learn more?
(Other definitions for veto that I've seen before include "Formally forbid" , "Authoritative prohibition" , "negative result" , "Reject - ban" , "Right to reject a decision" .)