Its riders are on board (4)
I believe the answer is:
surf
'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" .)