Showing two undergarments is most valiant (7)
I believe the answer is:
bravest
'most valiant' is the definition.
('brave' can be similar in meaning to 'valiant')
I don't understand the remainder of the clue.
(Other definitions for bravest that I've seen before include "Most daring" , "Most courageous" , "Most handsome" , "Most valorous" , "Most heroic" .)