Protest against idea (6)
I believe the answer is:
object
I believe this clue is a double definition.
'protest against' is the first definition.
(thesaurus)
'idea' is the second definition.
(I've seen this before)
(Other definitions for object that I've seen before include "oppose" , "design?" , "Make protest, disagree" , "Express dissent to the thing?" , "offer in opposition" .)