Free ticket? (6)

I think it could be either:

Which length description are you looking for?:

single

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This clue could be a double definition.

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

'ticket?' is the second definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

Can you help me to learn more?

(Other definitions for single that I've seen before include "Short pop record" , "Individual - unmarried" , "One-way ticket" , "Not married, just the one" , "Concentrating on only one aim" .)

I've seen this clue in the Evening Standard.

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