Many are to issue a challenge (4)
I believe the answer is:
dare
'issue a challenge' is the definition.
(I've seen this in another clue)
'many are' is the wordplay.
'many' becomes 'd' ().
'd'+'are'='DARE'
'to' acts as a link.
(Other definitions for dare that I've seen before include "Challenge, defy, face" , "Take the risk, face the danger" , "Challenge of courage" , "Risk; challenge" , "Be courageous enough to try or do" .)