Either one can go for anaesthetic (5)
I believe the answer is:
ether
'for anaesthetic' is the definition.
(ether is a kind of anaesthetic)
'either one can go' is the wordplay.
'one' becomes 'i' (Roman numeral).
'can go' suggests deleting specific letters (some letters go out of the word).
'either' with 'i' taken out is 'ETHER'.
(Other definitions for ether that I've seen before include "Liquid used as anaesthetic/ solvent" , "Colourless volatile highly inflammable liquid" , "Type of chemical" , "Once widely-used anaesthetic" , "Gas - air - space" .)