Tweet letter from Greek bishop in revolt (7)
I believe the answer is:
chirrup
'tweet' is the definition.
(I know that tweet can be written as chirrup)
'letter from greek bishop in revolt' is the wordplay.
'letter from greek' becomes 'chi' (letter of the Greek alphabet).
'bishop' becomes 'rr' (abbreviation for Right Reverend).
'in revolt' becomes 'up' (up can mean revolting or rioting).
'chi'+'rr'+'up'='CHIRRUP'
(Other definitions for chirrup that I've seen before include "Sound of small bird" , "Twitter" , "Sound made by a bird" , "Noise of a small bird" , "Make high-pitched sounds" .)