Valiant baritone's last seen in mixed choir (6)
I believe the answer is:
heroic
'valiant' is the definition.
(I've seen this in another clue)
'baritone's last seen in mixed choir' is the wordplay.
'last' suggests the final letters.
'seen in' indicates putting letters inside.
'mixed' indicates anagramming the letters.
The last letter of 'baritone' is 'e'.
'choir' with letters rearranged gives 'hroic'.
'e' inserted into 'hroic' is 'HEROIC'.
(Other definitions for heroic that I've seen before include "Supremely courageous and brave" , "Showing extreme courage" , "Superhuman (efforts)" , "larger than life" , "Showing no fear in the face of extreme difficulty" .)