International treaty is optimum (5)
I believe the answer is:
ideal
'optimum' is the definition.
(I've seen this before)
'international treaty' is the wordplay.
'international' becomes 'i' (abbreviation eg in IMF).
'treaty' becomes 'deal' (a deal is a treaty or pact).
'i'+'deal'='IDEAL'
'is' acts as a link.
(Other definitions for ideal that I've seen before include "Cutting/stabbing weapons" , "Just right" , "Perfect; visionary" , "highly desirable" , "Exemplar" .)