Unmarried? That's the ticket! (6)

I believe the answer is:

single

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'unmarried?' is the definition.
(I know that unmarried can be written as single)

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

Can you help me to learn more?

(Other definitions for single that I've seen before include "Just one - on one's own" , "one > individual" , "Free" , "One-way ticket" , "Individual - unmarried" .)

I've seen this clue in the Evening Standard.

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