Proof of purchase, bit held up by those starting rail trip (7)

I believe the answer is:

receipt

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

'proof of purchase' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for receipt that I've seen before include "Proof of purchase" , "Proof of payment made" , "Written acknowledgment of payment" , "Getting" , "Confirmation of purchase" .)

I've seen this clue in The Telegraph.

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.