500 are to be brave (4)
I believe the answer is:
dare
'brave' is the definition.
(I've seen this in another clue)
'500 are to be' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'are to' says to put letters next to each other.
'be' becomes 'are' (I've seen this in another clue).
'd'+'are'='DARE'
(Other definitions for dare that I've seen before include "Take the risk, face the danger" , "Dan - - , 'Eagle' pilot" , "Take a chance - read about it" , "Challenge of courage" , "Risk; challenge" .)