Fashionable cause (9)
I believe the answer is:
bandwagon
'fashionable cause' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bandwagon that I've seen before include "something fashionable all of a sudden" , "everybody wants to be on it" , "Suddenly fashionable cause or activity" , "Popular movement" , "vehicle for musicians" .)