In favour of a departure from defense alliance immediately (6)
I believe the answer is:
pronto
'immediately' is the definition.
(pronto can mean at once or quickly)
'in favour of a departure from' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'a departure' becomes 'nt' (I can't explain this - if you can you should give a lot more credence to this answer).
'from' becomes 'o' (I've seen this in other clues).
'pro'+'nt'+'o'='PRONTO'
'defense alliance' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for pronto that I've seen before include "Immediately, amigo" , "Quickly, without delay" , "Promptly, without delay" , "Right away, quickly" , "Double quick!" .)