Challenge aggressively (6)
I believe the answer is:
heckle
'challenge aggressively' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for heckle that I've seen before include "Interrupt (a public speaker) with taunts" , "Harass vocally" , "Jeer abusively" , "disrupt meeting" , "Shout abuse at, a speaker say" .)