There's no end to these pets (4,4)
I believe the answer is:
manx cats
'there's no end to these pets' is the definition.
The definition suggests an adverb but the answer is not.
'there's no end to these pets' is the wordplay.
'there's' says to put letters next to each other.
'no end' becomes 'manx' (I am not sure about this - if you are sure you should believe this answer much more).
'to these pets' becomes 'cats' (cat is a type of pet).
'manx' put next to 'cats' is 'MANX CATS'.
Can you help me to learn more?
(Another definition for manx cats that I've seen is " Tailless felines".)