Gesture: a wave, did you say? (4)
I believe the answer is:
sign
'gesture' is the definition.
(I know that sign is a more specific form of the action gesture)
'a wave did you say?' is the wordplay.
'a wave' becomes 'sine' (sine wave is a kind of wave).
'did you say?' indicates a 'sounds like' (homophone) clue.
'sine' sounds like 'SIGN'.
(Other definitions for sign that I've seen before include "Communicate to the deaf" , "Signal, gesture" , "Token - symbol - omen - write one's name" , "Pointer - indicate agreement in writing" , "Evidence" .)