Mammal - something necessary when housing revolutionary collection of animals (7)
I believe the answer is:
muskrat
'mammal' is the definition.
(muskrat is a kind of mammal)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for muskrat that I've seen before include "Vole-like creature" , "Beaver-like rodent with glossy brown fur" , "Rodent valued for its fur" , "UK, smart? (anag)" , "Large, smelly North American rodent" .)