“He’s only kidding,” you chirp (7)
I believe the answer is:
'chirp' is the definition.
(twittering is a kind of chirping)
'he's only kidding you' is the wordplay.
I cannot quite understand how this works, but
'he' could be 'wit' and 'wit' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for twitter that I've seen before include "networking service" , "Chirpy social website" , "Sound like small birds" , "Chatter" , "Electronic social network" .)