Reportedly prohibited joker having as topic fashionable movement (9)
I believe the answer is:
bandwagon
'fashionable movement' is the definition.
(I've seen this before)
'reportedly prohibited joker having as' is the wordplay.
'reportedly' indicates a 'sounds like' (homophone) clue.
'prohibited' becomes 'banned' (synonyms).
'joker' becomes 'wag' (I've seen this before).
'having as' becomes 'on' (I can't explain this - if you can you should believe this answer much more).
'banned' is a homophone of 'band'.
'band'+'wag'+'on'='BANDWAGON'
'topic' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for bandwagon that I've seen before include "Fashionable movement" , "Popular movement" , "Astaire/Buchanan film" , "everybody wants to be on it" , "something fashionable all of a sudden" .)