They travel fast and their return is celebrated (4)
I believe the answer is:
gnus
'they travel fast and their' is the definition.
The definition suggests an adverb but the answer is not.
'return is celebrated' is the wordplay.
'return' shows that the letters should be reversed in order.
'is celebrated' becomes 'sung' (as opposed to 'unsung').
'sung' reversed gives 'GNUS'.
Can you help me to learn more?
(Other definitions for gnus that I've seen before include "Wildebeests" , "Snug (anag) -- wildebeest" , "South African antelopes" , "Gun (anag.)" , "Big game" .)