Affected US soldier returning in a northern military action (8)
I believe the answer is:
campaign
'military action' is the definition.
(I know that campaign is a type of military action)
'affected us soldier returning in a northern' is the wordplay.
'affected' becomes 'camp' (camp can mean exaggerated or affected).
'us soldier' becomes 'GI' (GI can mean an American soldier).
'returning' says the letters should be written backwards.
'in' is an insertion indicator.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'gi' in reverse letter order is 'ig'.
'a'+'n'='an'
'ig' inserted within 'an' is 'aign'.
'camp'+'aign'='CAMPAIGN'
(Other definitions for campaign that I've seen before include "Promotion; crusade" , "Organised course of action" , "Series of battles (8)" , "Series of activities to attain a goal" , "Plan of action" .)