Alien contrives to secure a spell in the military (8,7)
I believe the answer is:
national service
'alien contrives' is the definition.
I can't tell whether this defines the answer.
'a spell in the military' is the wordplay.
'a spell in' becomes 'national' (I can't explain this - if you can you should give a lot more credence to this answer).
'the military' becomes 'service'.
'national'+'service'='NATIONAL SERVICE'
'to secure' acts as a link.
Can you help me to learn more?
(Other definitions for national service that I've seen before include "In a period with the military" , "Compulsory period in the forces" , "Period of compulsory conscription" , "What conscripts had to do" , "Period of peacetime conscription" .)