Choice of mild anaesthetic initially prolonged unconsciousness (4)
I believe the answer is:
coma
'prolonged unconsciousness' is the definition.
(I've seen this before)
'choice of mild anaesthetic initially' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'choice of mild anaesthetic' is 'COMA'.
(Other definitions for coma that I've seen before include "Unconscious state" , "Unconsciousness from which person cannot be aroused" , "Deep sleep" , "Deep trance" , "State of deep (often prolonged) unconsciousness" .)