Animal showing courage having lost energy (4)
I believe the answer is:
Here is my best explanation:
'animal' is the definition.
The answer is an example.
'courage having lost energy' is the subsidiary indication.
'courage' becomes 'heart' (heart is a kind of courage).
'having lost' indicates named letters should be taken away.
'energy' becomes 'e' (symbol in physics e.g. in Einstein's famous e=mc2).
'heart' with 'e' taken away is 'hart'.
'showing' is the link.
(Other definitions for hart that I've seen before include "Male red deer", "Male deer; Rodgers collaborator", "Songwriting partnership, Rodgers and --", "Richard Rodgers and Lorenz . . . . were famous musical partnership", "Stag", "Animal", "Mature male deer", "Beast", "animal" and "Male red-deer".)