Bird's home into a sorting aid (10)
I believe the answer is:
pigeonhole
'a sorting aid' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'bird's home' is the wordplay.
'bird' becomes 'pigeon' (type of bird).
'home' becomes 'hole' (I can't justify this - if you can you should believe this answer much more).
'pigeon'+'hole'='PIGEONHOLE'
'into' acts as a link.
Can you help me to learn more?
(Other definitions for pigeonhole that I've seen before include "Categorise" , "Classify; bird's home" , "Recess in loft" , "wherein one may put post" .)