Start to play with child's toy and chatter inanely (7)
I believe the answer is:
prattle
'chatter inanely' is the definition.
(I've seen this before)
'start to play with child's toy' is the wordplay.
'start to' indicates taking the first letters.
'with' is a charade indicator (letters next to each other).
'child's toy' becomes 'rattle' (I've seen this before).
The first letter of 'play' is 'p'.
'p'+'rattle'='PRATTLE'
'and' acts as a link.
(Other definitions for prattle that I've seen before include "Chatter inanely" , "Inconsequential chatter" , "Chatter inconsequentially, natter" , "Idle chatter" , "waffle" .)