Best treat for cat? (5)
I believe the answer is:
cream
This clue is a double definition.
'best' is the first definition.
(as in 'the cream of the crop')
'treat for cat?' is the second definition.
(I've seen this before)
(Other definitions for cream that I've seen before include "Elite; pale yellow" , "Elite; colour" , "Fat-rich part of milk" , "Top or best part" , "Best part of the milk" .)