Essentially animal sounds in addition (4)
I believe the answer is:
also
'in addition' is the definition.
(synonyms)
'essentially animal sounds' is the wordplay.
'essentially' says the answer is hidden in the clue (essence can mean the inside of something).
'ALSO' is hidden in the letters of 'animal sounds'.
(Other definitions for also that I've seen before include "on top" , "admitted to Royal Society > too" , "Item" , "in accompaniment" , "As well, in addition" .)