Love the sound of Cats as an alternative to opera (6)
I believe the answer is:
opuses
'an alternative to opera' is the definition.
('opera' is an alternative plural form for 'opus')
'love the sound of cats' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'the sound of' indicates a 'sounds like' (homophone) clue.
'cats' becomes 'pussies' (pussy is a kind of cat).
'pussies' is a homophone of 'puses'.
'o'+'puses'='OPUSES'
'as' is the link.
(Other definitions for opuses that I've seen before include "Musical works" , "pieces of music" .)