17 down
Travel
on
board
Chinook,
say,
seated
on
foam
(8)
Travel on board Chinook, say, seated on foam (8)
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The definition is at the beginning of the clue.
How the clue works
Right, so Travel on board is your definition, pointing to windsurfing. Now, a Chinook is a type of wind, and "say" is just flagging that up as an example, giving you WIND. Sit that on top of SURF (that's your foam), and you're seated on the board with WINDSURF.