Challenged many a socialist (5)
I believe the answer is:
dared
'challenged' is the definition.
(daring is a kind of challenging)
'many a socialist' is the wordplay.
'many' becomes 'd' ().
'socialist' becomes 'red' (red is a kind of socialist).
'd'+'a'+'red'='DARED'
(Other definitions for dared that I've seen before include "Was brave enough" , "Had the nerve to" , "Took a risk" , "was bold enough" , "Goaded" .)