Declares the donkey needs to rest anew (7)
I believe the answer is:
asserts
'declares' is the definition.
(asserting is a kind of declaring)
'the donkey needs to rest anew' is the wordplay.
'the donkey needs' becomes 'ass' (I can't justify this - if you can you should believe this answer much more).
'anew' is an anagram indicator.
'rest' with letters rearranged gives 'erts'.
'ass'+'erts'='ASSERTS'
Can you help me to learn more?
(Other definitions for asserts that I've seen before include "States categorically" , "Declares to be true" , "States firmly" , "States confidently and forcefully" .)