Cook cooks old birds (6)
I believe the answer is:
dodoes
'birds' is the definition.
(dodo is a kind of bird)
'cook cooks old' is the wordplay.
'cook' becomes 'do' (do can mean to cook).
'cooks old' becomes 'does' (I am not sure about this - if you are sure you should believe this answer much more).
'do'+'does'='DODOES'
Can you help me to learn more?
(Other definitions for dodoes that I've seen before include "people with little intelligence" , "Old-fashioned types" .)