Beach problem (7)
I believe the answer is:
erosion
'beach problem' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for erosion that I've seen before include "Gradual decline" , "Abrasion" , "being eaten away" , "Process of wearing away, by water or time" , "The wearing away by weather 97)" .)