The animals go back, dejected, to sleep (6)
I believe the answer is:
pandas
'the animals' is the definition.
The answer is an example.
'go back dejected to sleep' is the wordplay.
'go back' shows that the letters should be reversed in order.
'dejected' becomes 'sad' ('sad' is associated in meaning with 'dejected').
'to sleep' becomes 'nap' (napping is a kind of sleeping).
'sad'+'nap'='sadnap'
'sadnap' written backwards gives 'PANDAS'.
(Other definitions for pandas that I've seen before include "Large black and white mammals" , "Black and white Chinese bears" , "wild animals from Asia" , "police cars" , "zoo favourites" .)