Cooked bird - I ate a mouthful (8)
I believe the answer is:
diatribe
'a mouthful' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'cooked bird i ate' is the wordplay.
'cooked' indicates an anagram.
'bird'+'i'+'ate'='birdiate'
'birdiate' with letters rearranged gives 'DIATRIBE'.
Can you help me to learn more?
(Other definitions for diatribe that I've seen before include "Torrent of abuse" , "I debar it, being a violent attack in words" , "Insults" , "Bitter critical attack" , "Stream of invective" .)