Valiant energy in new choir (6)
I believe the answer is:
heroic
'valiant' is the definition.
(I've seen this before)
'energy in new choir' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'in' indicates putting letters inside.
'new' indicates anagramming the letters.
'choir' with letters rearranged gives 'hroic'.
'e' placed within 'hroic' is 'HEROIC'.
(Other definitions for heroic that I've seen before include "Superhuman (efforts)" , "Showing extreme courage" , "Very brave and noble" , "Supremely courageous and brave" , "Undaunted" .)