A stream hides crazy old animal (9)
I believe the answer is:
armadillo
'animal' is the definition.
(armadillo is a kind of animal)
'a stream hides crazy old' is the wordplay.
'stream' becomes 'rill' (rill is a kind of stream).
'hides' is an insertion indicator.
'crazy' becomes 'mad' (synonyms).
'old' becomes 'o' (abbreviation).
'rill' enclosing 'mad' is 'rmadill'.
'a'+'rmadill'+'o'='ARMADILLO'
(Other definitions for armadillo that I've seen before include "Burrowing mammal with a covering of bony plates" , "American insect-eater" , "bony individual?" , "mammal that has claws" , "Compete against, drama" .)