Councillor posed as rude old campaigner (8)
I believe the answer is:
crusader
'old campaigner' is the definition.
'crusader' can be an answer for 'campaigner' (eg someone on a moral crusade). I am not sure about the 'old' bit.
'councillor posed as rude' is the wordplay.
'councillor' becomes 'cr' (I've seen this in other clues).
'posed' indicates anagramming the letters (I've seen 'pose' mean this (the letters must be re-posed into a new order)).
'as'+'rude'='asrude'
'asrude' with letters rearranged gives 'usader'.
'cr'+'usader'='CRUSADER'
Can you help me to learn more?
(Other definitions for crusader that I've seen before include "For example, Richard I" , "Campaigner for a cause, like Richard the Lionheart" , "Champion" , "religious warrior" , "An old soldier" .)