Saw media manipulation across Britain (7)
I believe the answer is:
proverb
'saw' is the definition.
('proverb' can be a synonym of 'saw')
'media manipulation across britain' is the wordplay.
'media manipulation' becomes 'pr' (public relations).
'across' becomes 'over' ('over' can be a synonym of 'across').
'britain' becomes 'b' (common abbreviation as in GB).
'pr'+'over'+'b'='PROVERB'
(Other definitions for proverb that I've seen before include "Maxim" , "Old saw" , "Well-known statement incorporating general truth" , "Adage, pithy saying" , "Traditional saying" .)