Means of communication used by intermediaries (5)
I believe the answer is:
media
'means of communication' is the definition.
(mass media)
'used by intermediaries' is the wordplay.
'used by' indicates the answer is hidden within the clue.
'MEDIA' is hidden within 'intermediaries'.
(Other definitions for media that I've seen before include "Press, TV etc" , "TV networks and the like" , "reporting channels" , "Newspapers, TV, radio etc" , "means for spreading news" .)