One that stoops in autumn: short-tailed rook (6)
I believe the answer is:
falcon
'one that stoops' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'autumn short-tailed rook' is the wordplay.
'autumn' becomes 'fall' (synonyms).
'short' means to remove the last letter.
'tailed rook' becomes 'con' (synonyms. I am not sure about the 'tailed' bit.).
'fall' with its last letter taken away is 'fal'.
'fal'+'con'='FALCON'
'in' is the link.
Can you help me to learn more?
(Other definitions for falcon that I've seen before include "Hunting bird, may be peregrine" , "Kind of hunting bird - peregrine is one" , "A hunting bird, may be peregine" , "Long-winged hawk" , "that's seen flying" .)