Cook rings about Latvian opera (9)
I believe the answer is:
rigoletto
'opera' is the definition.
(I know that Rigoletto is a opera)
'cook rings about latvian' is the wordplay.
'cook' becomes 'rig' (I've seen this in other clues).
'rings' becomes 'oo' (multiple circles).
'about' means one lot of letters goes inside another.
'latvian' becomes 'Lett' (I've seen this in another clue).
'oo' enclosing 'lett' is 'oletto'.
'rig'+'oletto'='RIGOLETTO'
(Other definitions for rigoletto that I've seen before include "noted work" , "titled man" , "Verdi's hunchback jester" , "Opera by Verdi" , "court favourite" .)