Peak of a wave (5)
I believe the answer is:
crest
'peak of a wave' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for crest that I've seen before include "Heraldic device representing a family" , "Surmount" , "Badge" , "Top point of hill or wave" , "Wave top; (bird's) comb" .)