Animals vocalised on the way back (4)
I believe the answer is:
gnus
'animals' is the definition.
(I've seen this before)
'vocalised on the way back' is the wordplay.
'vocalised' becomes 'sung' (performed vocally).
'on the way back' says the letters should be written in reverse.
'sung' back-to-front is 'GNUS'.
(Other definitions for gnus that I've seen before include "South African antelopes" , "Gun (anag.)" , "Animals" , "runners in Africa" , "African runners" .)