Surfer's concerns (5)
I believe the answer is:
tides
'surfer's concerns' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tides that I've seen before include "Rises and falls, nautically" , "Lunar ocean movements" , "Current trends" , "Moon-driven ocean movements" , "Lunar-driven sea movements (5)" .)