Devil in protest, ending in heaven (5)
I believe the answer is:
demon
'devil' is the definition.
('demon' can be a synonym of 'devil')
'protest ending in heaven' is the wordplay.
'protest' becomes 'demo' (I've seen this before).
'ending in' says to take the final letters.
The final letter of 'heaven' is 'n'.
'demo'+'n'='DEMON'
'in' acts as a link.
(Other definitions for demon that I've seen before include "Evil spirit, devil" , "Imp" , "Inspiring force" , "Wizard" , "Bad spirit" .)