Surfer's concern (4)
I believe the answer is:
'surfer's concern' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tide that I've seen before include "Sea's ebb and flow" , "Rising and falling of the sea" , "Season; current" , "''There is a ... in the affairs of men'' (Julius Caesar)" , "Ebber and flower?" .)