Retreating, I left old, short rifle in military campaign (9)
I believe the answer is:
gallipoli
'military campaign' is the definition.
(I've seen this before)
'retreating i left old short rifle' is the wordplay.
'retreating' says the letters should be written in reverse.
'i left old' becomes 'ilo' (this could be a standard abbreviation which I don't know about).
'short' means to remove the last letter.
'rifle' becomes 'pillage' (both can mean to rob a place).
'pillage' with its final letter removed is 'pillag'.
'ilo'+'pillag'='ilopillag'
'ilopillag' back-to-front is 'GALLIPOLI'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for gallipoli that I've seen before include "military campaign" , "World War I battle of the Dardanelles" , "Turkish scene of World War One landing" , "Long ago landing" , "Turkish scene of WW1 landing" .)