Uplifting spread allowed such a communication (8)
I believe the answer is:
'a communication' is the definition.
(telegram is a kind of communication)
'uplifting spread allowed' is the wordplay.
'uplifting' shows that the letters should be reversed in order.
'spread' becomes 'marge' (marge is a kind of spread).
'allowed' becomes 'let' (to let is to allow or permit).
'margelet' reversed gives 'TELEGRAM'.
'such' acts as a link.
(Other definitions for telegram that I've seen before include "Electric message" , "Message transmitted by wire" , "cable" , "Pre-1980s electronic message" , "Dispatch" .)