See the registrar holding anaesthetic (5)
I believe the answer is:
ether
'anaesthetic' is the definition.
(ether is a kind of anaesthetic)
'see the registrar holding' is the wordplay.
'holding' says the answer is hidden in the clue.
'ETHER' is hidden within 'see the registrar'.
(Other definitions for ether that I've seen before include "Liquid used as anaesthetic/ solvent" , "Anaesthetic is, strangely, there" , "An early anaesthetic" , "Old anaesthetic -- three (anag)" , "There goes the anaesthetic" .)