Animals toned up in fresh air (8)
I believe the answer is:
rodentia
'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'.
(Other definitions for rodentia that I've seen before include "animals with strong teeth" , "Order including rats, mice, etc" , "natural order" .)