The favourite shows ill humour (3)
I believe the answer is:
pet
'the favourite' is the definition.
('pet' can be a synonym of 'favourite')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for pet that I've seen before include "Favourite or house animal" , "Domesticated animal" , "Tamed animal; sulk" , "Bad humour; darling" , "Favourite - maybe teacher's" .)