Carrier introduced when a treaty is arranged (3,4)
I believe the answer is:
tea tray
'carrier' is the definition.
(I have seen 'Domestic carrier ' mean 'tea tray' so perhaps 'carrier' could also mean 'tea tray')
'a treaty is arranged' is the wordplay.
'is arranged' indicates anagramming the letters (letters rearranged).
'a'+'treaty'='atreaty'
'atreaty' anagrammed gives 'TEA-TRAY'.
'introduced when' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tea tray that I've seen before include "Where service may be held" , "food transport" , "Delivery method for China" , "Domestic carrier" , "Pot, cups carrier" .)