International experiment with something in a box? (4,5)
I believe the answer is:
test match
'international' is the definition.
(cricket international)
'experiment with something in a box?' is the wordplay.
'experiment with' becomes 'test' (synonyms).
'something in a box?' becomes 'match' (something from a match box).
'test'+'match'='TEST MATCH'
(Other definitions for test match that I've seen before include "big game for some men?" , "Cricket contest between countries, usually one of a series" , "International affair" , "International meeting" , "International sports encounter" .)