Excellent trick for the audience in the interval (9)
I believe the answer is:
meanwhile
'the interval' is the definition.
(meanwhile is a kind of interval)
'excellent trick for the audience' is the wordplay.
'excellent' becomes 'mean' (I can't justify this - if you can you should believe this answer much more).
'trick' becomes 'wile' (wiles are tricks or deceptions).
'for the audience' indicates a 'sounds like' (homophone) clue (how an audience might hear it).
'wile' sounds like 'while'.
'mean'+'while'='MEANWHILE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for meanwhile that I've seen before include "in intervening period" , "For the moment" , "In the intervening time" , "On the other hand" , "In the interim" .)