Protection US soldier provided in the main, after retreating (5)
I believe the answer is:
aegis
'protection' is the definition.
(aegis can mean someone's guidance or protection)
'us soldier provided in the main after retreating' is the wordplay.
'us soldier' becomes 'GI' (GI can mean an American soldier).
'provided in' means one lot of letters goes inside another.
'the main' becomes 'sea' (the main is an old name for the sea).
'after retreating' says the letters should be written backwards.
'sea' back-to-front is 'aes'.
'gi' placed within 'aes' is 'AEGIS'.
(Other definitions for aegis that I've seen before include "strong defence" , "Kindly endorsement and guidance" , "Protection (originally of Zeus)" , "Protection (of someone)" , "Advocacy" .)