Wave, one that may be crashing? (4)
I believe the answer is:
bore
This clue is a double definition.
'wave' is the first definition.
(I've seen this before)
'one that may be crashing?' is the second definition.
(reference to the phrase 'crashing bore')
(Other definitions for bore that I've seen before include "Make a hole with a pointed tool" , "fatigue" , "Drill to be tiresome" , "Drill or be tedious" , "Fast-running water" .)