Unauthorised artwork revolutionary provided in strongly Labour newspaper (8)
I believe the answer is:
graffiti
'unauthorised artwork' is the definition.
'revolutionary provided in strongly labour newspaper' is the wordplay.
'revolutionary' is a reversal indicator.
'provided' becomes 'if'.
'in' indicates putting letters inside.
'strongly labour' becomes 'graft' (to graft is to work or labour hard).
'newspaper' becomes 'i' (UK newspaper).
'if' backwards is 'fi'.
'fi' put within 'graft' is 'graffit'.
'graffit'+'i'='GRAFFITI'
(Other definitions for graffiti that I've seen before include "spray-painting" , "Words or drawings scrawled on walls" , "Wall scribbles" , "Scribblings on the walls of buildings" , "Drawing" .)