Hat that's best on international (4)
I believe the answer is:
topi
'hat' is the definition.
(topi is a kind of hat)
'best on international' is the wordplay.
'best' becomes 'top' ('top' can be similar in meaning to 'best').
'on' says to put letters next to each other.
'international' becomes 'i' (abbreviation eg in IMF).
'top'+'i'='TOPI'
'that's' is the link.
(Other definitions for topi that I've seen before include "Large antelope; pith hat" , "Headgear" , "that goes ahead" , "African antelope" , "cover in the heat" .)