Bird starts getting out of sorts eventually (5)
I believe the answer is:
goose
'bird' is the definition.
(I know that goose is a type of domesticated bird)
'starts getting out of sorts eventually' is the wordplay.
'starts' suggests taking the first letters.
The initial letters of 'getting out of sorts eventually' is 'GOOSE'.
(Other definitions for goose that I've seen before include "Christmas bird?" , "Pimply state of the skin when cold or scared (with a bird)" , "Gander" , "Large water bird" , "Large waterbird" .)