Crossword Genius

Sort of sexual transport? (5)

Ross

I believe the answer is:

metro

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'sort of sexual transport?' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?

This is the entire clue.

Can you help me to learn more?

(Other definitions for metro that I've seen before include "Urban railway system" , "French underground" , "Paris underground rail system" , "Parisian underground system" , "what moves many Parisians" .)

I've seen this clue in The Guardian.
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