Petrol can be an anaesthetic (3)
I believe the answer is:
gas
This clue is a double definition.
'petrol' is the first definition.
('gas' can be a synonym of 'petrol')
'an anaesthetic' is the second definition.
'can be' is the link.
(Other definitions for gas that I've seen before include "See 2" , "Chatter (colloq.)" , "chew the fat" , "It's not liquid or solid, like American petrol" , "Talk idly; fuel" .)