Certain animal is unusually rude (7)
I believe the answer is:
assured
'certain' is the definition.
(I've seen this in another clue)
'animal is unusually rude' is the wordplay.
'animal' becomes 'ass' (term for a donkey).
'is unusually' indicates anagramming the letters.
'rude' is an anagram of 'ured'.
'ass'+'ured'='ASSURED'
(Other definitions for assured that I've seen before include "without doubt" , "Exhibiting confidence" , "Confident and self-possessed" , "Secured" , "Confident - certain" .)