It's alcoholic, so better leave out the teetotaller (4)
I believe the answer is:
beer
'teetotaller' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'it's alcoholic so better leave out' is the wordplay.
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'alcoholic so better leave' becomes 'ber' (I can't explain this - if you can you should give a lot more credence to this answer).
'out' indicates putting letters inside.
'e' inserted inside 'ber' is 'BEER'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for beer that I've seen before include "... this at pub?" , "Malt liquor" , "Drink brewed with malt" , "Ale, lager etc" , "Drink using hops, malt, etc" .)