Both uninitiated, Stacy and Bella mounted animal (5,3)

I believe the answer is:

alley cat

'animal' is the definition.

I don't understand the rest of the clue.

(Other definitions for alley cat that I've seen before include "Stray mog" , "Stray urban feline" , "Homeless feline" , "City feline that prowls the back lanes" , "homeless creature" .)

I've seen this clue in The Independent.

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