Message spread, allowed to be written about (8)
I believe the answer is:
telegram
'message' is the definition.
(telegram is a kind of message)
'spread allowed to be written about' is the wordplay.
'spread' becomes 'marge' (marge is a kind of spread**).
'allowed' becomes 'let' (I've seen this before).
'to be written about' says the letters should be written in reverse.
'marge'+'let'='margelet'
'margelet' reversed gives 'TELEGRAM'.
(Other definitions for telegram that I've seen before include "Message sent by wire" , "Message once sent by radio or electrical signals" , "cable" , "(Old) type of message" , "Pre-1980s electronic message" .)