Animals toned up in fresh air (8)
I believe the answer is:
Here is my best explanation:
'animals' is the definition.
(I've seen this before)
'toned up in fresh air' is the wordplay.
'up' indicates an anagram.
'in' is an insertion indicator.
'fresh' is an anagram indicator.
'toned' is an anagram of 'odent'.
'air' anagrammed gives 'ria'.
'odent' inserted into 'ria' is 'RODENTIA'.