Challenging attorney to call (6)
I believe the answer is:
daring
'challenging' is the definition.
(daring is a kind of challenging)
'attorney to call' is the wordplay.
'attorney' becomes 'DA' (District Attorney).
'to call' becomes 'ring' (telephone call).
'da'+'ring'='DARING'
(Other definitions for daring that I've seen before include "Derring-do" , "Impudent - brave - courage" , "'Adventurous, bold (6)'" , "Courageous" , "Boldly dramatic or unconventional" .)