Fiendish protest event, not entirely pleasant (7)
I believe the answer is:
demonic
'fiendish' is the definition.
(I know that fiendish can be written as demonic)
'protest event not entirely pleasant' is the wordplay.
'protest event' becomes 'demo' (demonstration).
'not entirely' means to remove the last letter (not all of the word).
'pleasant' becomes 'nice' (similar in meaning).
'nice' with its last letter taken off is 'nic'.
'demo'+'nic'='DEMONIC'
(Other definitions for demonic that I've seen before include "Furious" , "Resembling evil spirits" , "Wicked" , "Dime con (anag.)" , "Devilish" .)