Deter assembly after fool chirped (9)
I believe the answer is:
twittered
'chirped' is the definition.
(twittering is a kind of chirping)
'deter assembly after fool' is the wordplay.
'assembly' is an anagram indicator.
'after' is a charade indicator (letters next to each other).
'fool' becomes 'twit' (both can mean a stupid person).
'deter' anagrammed gives 'tered'.
'tered' put after 'twit' is 'TWITTERED'.
(Other definitions for twittered that I've seen before include "sang like a bird!" , "said nothing of value" , "Chirped away" , "Sounded like a small bird" .)