Knock back means processed cheese (8)
I believe the answer is:
parmesan
'cheese' is the definition.
(Parmesan is an example)
'knock back means processed' is the wordplay.
'knock' becomes 'rap' ('rap' can be a synonym of 'knock').
'back' is a reversal indicator.
'processed' indicates anagramming the letters.
'rap' back-to-front is 'par'.
'means' with letters rearranged gives 'mesan'.
'par'+'mesan'='PARMESAN'
(Other definitions for parmesan that I've seen before include "Cheese for grating" , "Dry sharp-flavoured Italian cheese" , "Hard dry Italian cheese" , "Italian 8 across" , "that grates" .)