Mostly average means of mass communication (5)
I believe the answer is:
media
'means of mass communication' is the definition.
(I've seen this before)
'mostly average' is the wordplay.
'mostly' means to remove the last letter.
'average' becomes 'median' (median is a kind of average).
'median' with its last letter removed is 'MEDIA'.
(Other definitions for media that I've seen before include "Mass communications" , "sources of information" , "oils as examples" , "Purveyors of information to the public - I made them" , "Newspapers, TV, radio etc" .)