Top of a wave (5)
I believe the answer is:
crest
'top of a wave' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for crest that I've seen before include "Top point of hill or wave" , "Wave top; (bird's) comb" , "Crown, peak, summit" , "Peak - comb" , "Top (of wave etc)" .)