Aorta and lung repaired to keep one in US force (8,5)
I believe the answer is:
national guard
'us force' is the definition.
(the National Guard is a US military force)
'aorta and lung repaired to keep one' is the wordplay.
'repaired' indicates anagramming the letters.
'to keep' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'aorta'+'and'+'lung'='aortaandlung'
'aortaandlung' anagrammed gives 'natonalguard'.
'natonalguard' placed around 'i' is 'NATIONAL GUARD'.
'in' acts as a link.
(Another definition for national guard that I've seen is " US military reserve force".)