Host exulted, exuding energy (5)
I believe the answer is:
crowd
'host' is the definition.
(I've seen this in another clue)
'exulted exuding energy' is the wordplay.
'exulted' becomes 'crowed' (crow can mean boast of or exult in).
'exuding' is a deletion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'crowed' with 'e' removed is 'CROWD'.
(Other definitions for crowd that I've seen before include "Throng" , "Rabble" , "Large number of people gathered together" , "Army" , "Mob of people" .)