Birds which need some bigger nests (5)
I believe the answer is:
ernes
'birds' is the definition.
(erne is a kind of bird)
'need some bigger nests' is the wordplay.
'need some' says the answer is hidden in the clue (I've seen 'needing some' mean this).
'ERNES' is hidden in the letters of 'bigger nests'.
'which' is the link.
(Other definitions for ernes that I've seen before include "Certain birds" , "Irish lough's" , "They fly across the sea" , "Sea-eagles" , "sea eagles" .)