Very hot always inside cold-blooded animal (8)
I believe the answer is:
feverish
'very hot' is the definition.
'always inside cold-blooded animal' is the wordplay.
'always' becomes 'ever' (synonyms).
'inside' is an insertion indicator.
'cold-blooded animal' becomes 'fish' (fish is a kind of animal. I am not sure about the 'cold-blooded' bit.).
'ever' put within 'fish' is 'FEVERISH'.
Can you help me to learn more?
(Other definitions for feverish that I've seen before include "in a tizz" , "Restlessly excited" , "Nervously excited" , "Frenzied" , "Restless with excitement or agitation - got ague?" .)