Ride on and on at Longchamp, then start to flag (4)
I believe the answer is:
surf
'ride on' is the definition.
(as in riding or surfing on a wave)
'on at longchamp then start to flag' is the wordplay.
'on at longchamp' becomes 'sur' ('on' in French - Longchamp is in France).
'then' says to put letters next to each other.
'start to' says to take the initial letters.
The initial letter of 'flag' is 'f'.
'sur'+'f'='SURF'
'and' is the link.
(Other definitions for surf that I've seen before include "Sea foam" , "Waves for riding" , "Sea-foam" , "Breaking sea" , "Foam formed by breaking waves" .)