As fish may be, but not bird or mammal (6)
I believe the answer is:
plural
'as' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for plural that I've seen before include "Numbering more than one" , "Word form denoting more than one is involved" , "Word usually ending in 's'" , "Words such as "sheep" and "deer" (but not "cat" and "dog") can be this" , "Composed of more than one" .)