Wearing old saucy cape, Joker finds authority questioned in court (6,7)
I believe the answer is:
expert witness
'wearing old saucy cape joker' is the definition.
The definition suggests a singular noun which matches the answer.
'authority questioned in court' is the wordplay.
'authority' becomes 'expert' (I've seen this before).
'questioned in court' becomes 'witness' (I can't justify this - if you can you should believe this answer much more).
'expert'+'witness'='EXPERT WITNESS'
'finds' acts as a link.
Can you help me to learn more?
(Other definitions for expert witness that I've seen before include "Court specialist" , "One providing specialist testimony" , "Wiser next step? (anag)" .)