Conservative regret about pathetic campaign (7)
I believe the answer is:
crusade
'campaign' is the definition.
(I know that Crusade is a type of expedition)
'conservative regret about pathetic' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'regret' becomes 'rue' (to rue something is to regret it).
'about' indicates putting letters inside.
'pathetic' becomes 'sad' (I've seen this in another clue).
'rue' going around 'sad' is 'rusade'.
'c'+'rusade'='CRUSADE'
(Other definitions for crusade that I've seen before include "Used car involved in campaign for a cause" , "Vigorous campaign" , "Holy war" , "Advertising campaign" , "Drive" .)