Wildebeests whose return is celebrated (4)
I believe the answer is:
gnus
'wildebeests' is the definition.
('gnu' can be a synonym of 'wildebeest')
'return is celebrated' is the wordplay.
'return' says the letters should be written in reverse.
'is celebrated' becomes 'sung' (as opposed to 'unsung').
'sung' back-to-front is 'GNUS'.
'whose' acts as a link.
(Other definitions for gnus that I've seen before include "Snug (anag) -- wildebeest" , "Big game" , "Large African antelopes" , "African runners" , "Animals" .)