Be cruel to favourite dog? (7)
I believe the answer is:
whippet
'dog?' is the definition.
(whippet is a kind of dog)
'be cruel to favourite' is the wordplay.
'be cruel to' becomes 'whip' (I can't explain this - if you can you should believe this answer much more).
'favourite' becomes 'pet' (synonyms).
'whip'+'pet'='WHIPPET'
Can you help me to learn more?
(Other definitions for whippet that I've seen before include "Dog similar to a greyhound" , "Fast runner" , "Dog related to 9 down" , "racing animal" , "Small greyhound-type dog" .)