Wave very gently, upset about that (6)
I believe the answer is:
ripple
'wave' is the definition.
(I know this)
'very gently upset about that' is the wordplay.
'very gently' becomes 'pp' (musical abbreviation for pianissimo, 'very softly').
'upset' becomes 'rile' (I've seen this in another clue).
'about that' indicates putting letters inside.
'pp' inserted inside 'rile' is 'RIPPLE'.
(Other definitions for ripple that I've seen before include "Small wave or surface disturbance" , "Slight disturbance on surface of lake" , "Slight disturbance on water's surface" , "Small wave on a liquid's surface" , "Tiny wave; gentle applause" .)