Silver not returned to South Pacific kingdom (5)
I believe the answer is:
tonga
'south pacific kingdom' is the definition.
(I've seen this before)
'silver not returned' is the wordplay.
'silver' becomes 'ag' ('Ag' can be a synonym of 'silver').
'returned' shows that the letters should be reversed in order.
'ag'+'not'='agnot'
'agnot' backwards is 'TONGA'.
'to' is the link.
(Other definitions for tonga that I've seen before include "Two-wheeled Indian vehicle" , "Pacific Ocean monarchy" , "South Pacific kingdom, capital Nuku'alofa" , "Horse-drawn vehicle" , "Light vehicle in India" .)