One helping the cast dance and become exhausted, losing some energy (8)
I believe the answer is:
prompter
'one helping the cast' is the definition.
(a prompter might give actors their lines)
'dance and become exhausted losing some energy' is the wordplay.
'dance' becomes 'prom' (prom is a kind of dance).
'and' says to put letters next to each other.
'become exhausted' becomes 'peter' (as in petering out).
'losing' indicates named letters should be taken away.
'some energy' becomes 'e' (physics symbol as in E=mcr).
'peter' with 'e' removed is 'pter'.
'prom'+'pter'='PROMPTER'
(Other definitions for prompter that I've seen before include "to be ready earlier" , "Helper of actors' memory" , "One helping actors - much quicker" , "Supplier of lines in the theatre" , "Reminder" .)