Crusade Conservative politician is entering again, sadly (8)
I believe the answer is:
campaign
'crusade' is the definition.
('campaign' can be a synonym of 'crusade')
'conservative politician is entering again sadly' is the wordplay.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'politician' becomes 'mp'.
'is entering' indicates putting letters inside.
'sadly' is an anagram indicator.
'again' anagrammed gives 'aaign'.
'mp' put into 'aaign' is 'ampaign'.
'c'+'ampaign'='CAMPAIGN'
(Other definitions for campaign that I've seen before include "Am pacing around in series of operations, perhaps military" , "Series of activities to attain a goal" , "Co-ordinated activities for a purpose" , "Series of military operations intended to achieve a goal" , "A series of planned activities, as for advertising" .)