Implore the registrar withholding anaesthetic drug (5)
I believe the answer is:
ether
'drug' is the definition.
(ether is a kind of drug)
'implore the registrar withholding anaesthetic' is the wordplay.
'ETHER' is hidden within the letters.
But, I'm uncertain how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for ether that I've seen before include "It could knock you out" , "number" , "Clear upper air" , "Former anaesthetic" , "Some were put out by this" .)