Favourites slowly walk up (4)
I believe the answer is:
pets
'favourites' is the definition.
('pet' can be a synonym of 'favourite')
'walk up' is the wordplay.
'walk' becomes 'step' (I've seen this before).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'step' written backwards gives 'PETS'.
'slowly' acts as a link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for pets that I've seen before include "Domestic animals" , "Little darlings - step on them" , "Animals at home" , "Domesticated animals" .)