I excluded one or the other anaesthetic (5)
I believe the answer is:
ether
'anaesthetic' is the definition.
(ether is a kind of anaesthetic)
'i excluded one or the other' is the wordplay.
I cannot really understand how this works, but
'one' could be 'the' (both can introduce a definite example of something) and 'the' is found within the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ether that I've seen before include "Colourless volatile highly inflammable liquid" , "Light liquid used as anaesthetic" , "Clear upper air" , "Gas - air - space" , "Colourless liquid used as a solvent" .)