Transatlantic alliance has power to finish things (3,2)
I believe the answer is:
use up
'finish things' is the definition.
'use up' can be an answer for 'finish' (to use something up is to finish it off). I'm not sure about the 'things' bit.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for use up that I've seen before include "Completely consume" , "Entirely consume" , "Exhaust (resources)" , "Expend" , "Consume (stocks)" .)