I believe the answer is:
'undulation' is the definition.
(I know that undulation can be written as wave)
This is the entire clue.
(Other definitions for wave that I've seen before include "may have a crest at the top?" , "Shake a hand in the air" , "Undulating curve" , "Move hand to and fro as greeting" , "60s cinematic movement with 1a [NEW]" .)