In such a cage, some birds would rise! (3)
I believe the answer is:
rib
'in such a cage' is the definition.
I can't judge whether this definition defines the answer.
'some birds would rise' is the wordplay.
'some' means to look at the middle letters.
'would rise' becomes 'ib' (I can't explain this - if you can you should believe this answer much more).
The middle letter of 'birds' is 'r'.
'r'+'ib'='RIB'
Can you help me to learn more?
(Other definitions for rib that I've seen before include "Curved bone joined to the spine" , "Thin bone" , "Part of vault" , "Chest bone, may be floating" , "Slender bone" .)