One would spout “Down with Prince” with energy (4,5)
I believe the answer is:
blue whale
'one would spout' is the definition.
I can't judge whether this definition defines the answer.
'down with prince with energy' is the wordplay.
'down' becomes 'blue' (both can mean sad).
'with' becomes 'w' (abbreviation).
'prince with' becomes 'hal' (I can't explain this - if you can you should believe this answer much more).
'energy' becomes 'e' (physics symbol as in E=mca).
'blue'+'w'+'hal'+'e'='BLUE WHALE'
(Other definitions for blue whale that I've seen before include "there's nothing bigger" , "Largest living animal" , "World's largest mammal" , "World's largest animal" .)