One spotted sign of expected standard on top of door? (7)

I believe the answer is:
leopard
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'sign of expected standard on top of door?' is the wordplay.
'sign' becomes 'leo' (star sign).
'of expected standard' becomes 'par'.
'on top of' indicates taking the first letters.
The initial letter of 'door' is 'd'.
'leo'+'par'+'d'='LEOPARD'
'spotted' is the link.
Can you help me to learn more?
(Other definitions for leopard that I've seen before include "The cat that does not change its spots" , "Spotted feline" , "Animal that doesn't change its spots" , "dangerous when free" , "Powerful cat" .)
