Number in Dorset he rang? (5)
I believe the answer is:
ether
'number' is the definition.
(anaesthetic, something which numbs)
'in dorset he rang?' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ETHER' is hidden in the letters of 'dorset he rang'.
(Other definitions for ether that I've seen before include "Volatile organic liquid once used as a general anaesthetic" , "Anaesthetic liquid; clear sky" , "Rare atmosphere" , "Medium supposedly" , "Colourless volatile highly inflammable liquid" .)