Orders cat's diet to be changed (8)
I believe the answer is:
dictates
'orders' is the definition.
('dictate' can be a synonym of 'order')
'cat's diet to be changed' is the wordplay.
'to be changed' is an anagram indicator.
'cats'+'diet'='catsdiet'
'catsdiet' anagrammed gives 'DICTATES'.
(Other definitions for dictates that I've seen before include "Orders" , "Gives imperative command in acid test" .)