Leaders of any truly international show (6)
I believe the answer is:
attest
'show' is the definition.
(attesting is a kind of showing)
'leaders of any truly international' is the wordplay.
'leaders of' suggests taking the first letters.
'international' becomes 'test' (I've seen this before).
The initial letters of 'any truly' is 'at'.
'at'+'test'='ATTEST'
(Other definitions for attest that I've seen before include "Bear formal witness to, give evidence" , "Give evidence in a court of law" , "Certify or bear witness" , "Demonstrate" , "Confirm to be true" .)