Animal trainer excited about source of protein (8)
I believe the answer is:
terrapin
'animal' is the definition.
The answer is an example.
'trainer excited about source of protein' is the wordplay.
'excited' indicates an anagram.
'about' indicates putting letters inside.
'source of' suggests taking the first letters.
The initial letter of 'protein' is 'p'.
'trainer' anagrammed gives 'terrain'.
'terrain' placed around 'p' is 'TERRAPIN'.
(Other definitions for terrapin that I've seen before include "Give out" , "Small species of turtle" , "Little turtle" , "Hardback" , "A reptile; a printer (anag.)" .)