Travel card: one's told to validate it (6)

I believe the answer is:

ticket

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'travel card' is the definition.
'ticket' can be an answer for 'card' (thesaurus). I'm not certain of the 'travel' bit.

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

Can you help me to learn more?

(Other definitions for ticket that I've seen before include "Permit to do something" , "Parking offence notice" , "Party's election policies; label" , "It entitles the bearer, to enter perhaps" , "Authority to travel" .)

I've seen this clue in The Guardian.

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