District Attorney referring to challenge (4)
I believe the answer is:
dare
'challenge' is the definition.
(daring is a kind of challenging)
'district attorney referring to' is the wordplay.
'district attorney' becomes 'da' ('D.A.' can be a synonym of 'district attorney').
'referring to' becomes 're' (**).
'da'+'re'='DARE'
(Other definitions for dare that I've seen before include "Have the courage to try something" , "Venture to read" , "Be bold enough (to)" , "Be brave enough" , "Have sufficient courage" .)