Animal showing courage having lost energy (4)
I believe the answer is:
hart
'animal' is the definition.
(I've seen this before)
'courage having lost energy' is the wordplay.
'courage' becomes 'heart' (heart is a kind of courage).
'having lost' indicates named letters should be taken away.
'energy' becomes 'e' (physics symbol as in E=mc2).
'heart' with 'e' removed is 'HART'.
'showing' acts as a link.
(Other definitions for hart that I've seen before include "providing material for Richard Rodgers?" , "Stag" , "Richard Rodgers and Lorenz . . . . were famous musical partnership" , "Deer; Lorenz --, lyricist" , "Beast" .)