When a teetotaler enters to make purchase, it’s a sandwich (5)

I believe the answer is:

butty

I'm a little stuck... Click here to teach me more about this clue!

'sandwich' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for butty that I've seen before include "Sandwich; towed barge" , "Sandwich; freight barge" , "something to eat" , "Partner" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.