Sound of wave motion (4)
I believe the answer is:
sign
'motion' is the definition.
(I know that sign is a type of motion)
'sound of wave' is the wordplay.
'sound of' indicates a 'sounds like' (homophone) clue.
'wave' becomes 'sine wave' (sine wave is a kind of wave).
'sine' is a homophone of 'SIGN'.
(Other definitions for sign that I've seen before include "Endorse" , "Leo, for instance" , "Communicate to the deaf" , "Autograph" , "Traffic control - to write one's name" .)