Giggle about W being X's predecessor (7)
I believe the answer is:
'giggle' is the definition.
(thesaurus)
'about w being x's predecessor' is the wordplay.
'about' means one lot of letters goes inside another.
'being x's predecessor' becomes 'titter' (I can't justify this - if you can you should give a lot more credence to this answer).
'w' placed into 'titter' is 'TWITTER'.
Can you help me to learn more?
(Other definitions for twitter that I've seen before include "Chirping (could be electronic!)" , "Chirpy social website" , "Make high-pitched sounds, as of birds" , "social network site revealed" , "Chirrup tremulously" .)