Buñuel has one to relieve revolutionary reviewer (9)
I believe the answer is:
diacritic
'bunuel has one' is the definition.
(the n in Buuel has a diacritic on it)
'relieve revolutionary reviewer' is the wordplay.
'relieve revolutionary' becomes 'dia' (I am not sure about this - if you are sure you should believe this answer much more).
'reviewer' becomes 'critic' (I've seen this before).
'dia'+'critic'='DIACRITIC'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for diacritic that I've seen before include "[ACCENT] , perhaps" , "Capable of distinguishing" , "accent, for example" , "Mark" , "grave perhaps" .)