Pacing, troubled about a mass crusade (8)
I believe the answer is:
campaign
'crusade' is the definition.
(I know that crusade is a more specific form of the action campaign)
'pacing troubled about a mass' is the wordplay.
'troubled' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'pacing' is an anagram of 'cpaign'.
'a'+'m'='am'
'cpaign' going around 'am' is 'CAMPAIGN'.
(Other definitions for campaign that I've seen before include "A series of planned activities, as for advertising" , "Electioneer" , "Am pacing around in series of operations, perhaps military" , "electioneering" , "Am pacing about in series of planned activities" .)