Unmarried? That's the ticket! (6)
I believe the answer is:
single
'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" .)