Putting pressure on the alliance, that’s the custom (6)
I believe the answer is:
praxis
'the custom' is the definition.
(thesaurus)
'putting pressure on the alliance' is the wordplay.
'putting pressure on' becomes 'pr' (I can't explain this - if you can you should give a lot more credence to this answer).
'the alliance' becomes 'axis' (alliance between two countries).
'pr'+'axis'='PRAXIS'
'that's' is the link.
Can you help me to learn more?
(Other definitions for praxis that I've seen before include "Working out of theory" , "Application of theory" , "accepted custom" , "Practice, as distinct from theory, of an art or science" .)