Challenged person prosecuting left-winger (5)
I believe the answer is:
dared
'challenged' is the definition.
(daring is a kind of challenging)
'person prosecuting left-winger' is the wordplay.
'person prosecuting' becomes 'da' (district attorney).
'left-winger' becomes 'red' (red is a kind of left-winger).
'da'+'red'='DARED'
(Other definitions for dared that I've seen before include "Acted with courage - or dread?" , "Challenged - was audacious" , "Goaded" , "was bold enough" , "Challenged (to a deed)" .)