What passengers pay to get on? (4)

I believe the answer is:

fare

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'to get on?' is the definition.
(I've seen this in another clue)

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

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(Other definitions for fare that I've seen before include "going rate?" , "Taxi hire cost" , "what the commuter will pay" , "diet" , "Price of passage" .)

I've seen this clue in the Evening Standard.

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