Stephen Spender's caged birds (4)
I believe the answer is:
hens
'birds' is the definition.
(hen is a type of bird)
'stephen spender's caged' is the wordplay.
'caged' indicates a hidden word.
'HENS' is hidden in the letters of 'stephen spenders'.
(Other definitions for hens that I've seen before include "Female chickens" , "Female birds" , "Poultry" , "Adult chickens" , "Birds kept for laying" .)