Animals celebrated, rolling over (4)
I believe the answer is:
gnus
'animals' is the definition.
(I've seen this before)
'celebrated rolling over' is the wordplay.
'celebrated' becomes 'sung' (as opposed to 'unsung').
'rolling over' shows that the letters should be reversed in order.
'sung' written backwards gives 'GNUS'.
(Other definitions for gnus that I've seen before include "South African antelopes" , "Snug (anag) -- wildebeest" , "Gun (anag.)" , "Wildebeests" , "runners in Africa" .)