Periodically they'll spread revolutionary message (8)

I believe the answer is:

'message' is the definition.
(telegram is a kind of message)

'periodically they'll spread revolutionary' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'spread' becomes 'marge' (marge is a kind of spread**).
'revolutionary' says the letters should be written backwards.
The alternating letters of 'theyll' are 'tel'.
'marge' reversed gives 'egram'.
'tel'+'egram'='TELEGRAM'

(Other definitions for telegram that I've seen before include "Message sent by wire" , "It may bring word" , "Message once sent by radio or electrical signals" , "Now obsolete communication over a wire" , "Pre-1980s electronic message" .)

I've seen this clue in The Independent.

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