Animals returning earth's nitrogen content (5)
I believe the answer is:
lions
'animals' is the definition.
The answer is an example.
'returning earth's nitrogen content' is the wordplay.
'returning' says the letters should be written backwards.
'earth' becomes 'soil' (earth can mean soil or dirt).
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'content' indicates putting letters inside.
'soil' enclosing 'n' is 'snoil'.
'snoil' back-to-front is 'LIONS'.
(Other definitions for lions that I've seen before include "Large carnivores" , "Animals in pride" , "Roaring cats" , "Large cats" , "Wild cats" .)