Rome's authority imposes tax on international tin (7)
I believe the answer is:
vatican
'rome's authority imposes' is the definition.
I can't judge whether this definition defines the answer.
'tax on international tin' is the wordplay.
'tax' becomes 'vat' (British sales tax).
'on' says to put letters next to each other.
'international' becomes 'i' (abbreviation eg in IMF).
'tin' becomes 'can' ('can' can be a synonym of 'tin').
'vat'+'i'+'can'='VATICAN'
Can you help me to learn more?
(Other definitions for vatican that I've seen before include "religious authority" , "Papacy" , "See City" , "City within Rome" , "European state" .)