Birds, hens or cuckoo? (6)
I believe the answer is:
herons
'birds' is the definition.
(heron is a kind of bird)
'hens or cuckoo?' is the wordplay.
'cuckoo?' indicates an anagram (I've seen 'cuckoo' mean this).
'hens'+'or'='hensor'
'hensor' anagrammed gives 'HERONS'.
(Other definitions for herons that I've seen before include "leggy birds" , "Waders" , "Wading birds with long legs" .)