Faction's ostentatiousness (4)
I believe the answer is:
side
I believe this clue is a double definition.
'faction's' is the first definition.
(one side in a conflict or controversy)
'ostentatiousness' is the second definition.
(side can mean ostentatiousness or pretension)
(Other definitions for side that I've seen before include "group of players" , "Team - arrogance" , "Accompanying" , "part" , "Airs, conceit" .)