Comedy routine's quiet moment (6)
I believe the answer is:
shtick
'comedy routine's' is the definition.
(I've seen this before)
'quiet moment' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'moment' becomes 'tick' (both can mean a very short amount of time).
'sh'+'tick'='SHTICK'
(Other definitions for shtick that I've seen before include "comedy routine" , "comic's routine" , "funny routine" , "Performer's routine" , "Comedian's routine - kitsch (anag)" .)