Appearing in animal sounds too (4)
I believe the answer is:
also
'too' is the definition.
(synonyms)
'appearing in animal sounds' is the wordplay.
'appearing in' indicates a hidden word.
'ALSO' is hidden within 'animal sounds'.
(Other definitions for also that I've seen before include "As well, in addition" , "on top" , "Old" , "Item" , "'Besides, moreover (4)'" .)