Dandruff in animal's hair occurring twice (6)
I believe the answer is:
'dandruff in animal's' is the definition.
'furfur' can be an answer for 'dandruff' (thesaurus). I'm unsure of the 'in animal's' bit.
'hair occurring twice' is the wordplay.
'hair' becomes 'fur' (fur is a kind of hair).
'occurring twice' becomes 'fur' (I am not sure about this - if you are sure you should believe this answer much more).
Can you help me to learn more?
(Another definition for furfur that I've seen is " dander".)