What a surfer surfs on (4)
I believe the answer is:
wave
'what a surfer surfs on' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for wave that I've seen before include "roller, perhaps" , "It may be permanent or Mexican" , "Move hand to and fro as greeting" , "Hand signal; surge" , "may have a crest at the top?" .)