Category includes good cat (5)
I believe the answer is:
tiger
'cat' is the definition.
(I know that tiger is a type of cat)
'category includes good' is the wordplay.
'category' becomes 'tier' (I've seen this in another clue).
'includes' means one lot of letters goes inside another.
'good' becomes 'g' (abbreviation).
'tier' placed around 'g' is 'TIGER'.
(Other definitions for tiger that I've seen before include "Dynamic type - striped animal" , "Asian animal" , "audacious person" , "Fierce person" , "Predator" .)