Gesture - sort of wave, say (4)
I believe the answer is:
sign
'gesture' is the definition.
(sign is a kind of gesture)
'sort of wave say' is the wordplay.
'sort of wave' becomes 'sine'.
'say' shows a homophone (sound like).
'sine' sounds like 'SIGN'.
(Other definitions for sign that I've seen before include "Indication; symbol" , "Pointer - indicate agreement in writing" , "house" , "Ram" , "As the star was for 14 across" .)