He's making fun of the bird imitation (7)
I believe the answer is:
'bird imitation' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'he's making fun of the' is the wordplay.
I cannot quite see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
'he' could be 'wit' and 'wit' is found within the answer.
The remaining letters 'ter' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for twitter that I've seen before include "Social media provider" , "Sound made by bird" , "(Make) a noise like a bird" , "Make bird noises" , "Bird's sound; trivial talk" .)