Cameron briefly discussed agony of political effort (8)
I believe the answer is:
'effort' is the definition.
(I know that effort can be written as campaign)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for campaign that I've seen before include "Military operation" , "battle" , "lobby" , "offensive" , "Series of planned actions to achieve a goal" .)