Kit and Colleen regularly over-the-top about opera (9)
I believe the answer is:
rigoletto
'opera' is the definition.
(I know that Rigoletto is a opera)
'kit and colleen regularly over-the-top about' is the wordplay.
'kit' becomes 'rig' (I've seen this before).
'and' means one lot of letters go next to another.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'over-the-top' becomes 'OTT' (abbreviation).
'about' is a reversal indicator.
The alternate letters of 'colleen' are 'ole'.
'ott' written backwards gives 'tto'.
'rig'+'ole'+'tto'='RIGOLETTO'
(Other definitions for rigoletto that I've seen before include "noted work" , "Verdi's hunchback jester" , "titled man" , "Opera by Verdi" , "court favourite" .)