Crossword Genius

Its riders are on board (4)

Ross

I believe the answer is:

surf

I'm a little stuck... Click here to teach me more about this clue!

'board' is the definition.
The definition and answer can be both to do with motion as well as being verbs in their base form.
Perhaps there's a link between them I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for surf that I've seen before include "Seaside froth" , "Foaming waves" , "Channel-hop" , "White foam of breaking waves" , "Ride a wave" .)

I've seen this clue in the Evening Standard.
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