Saviour of old Thailand briefly trapped in net (7)
I believe the answer is:
messiah
'saviour' is the definition.
(messiah is a kind of saviour)
'old thailand briefly trapped in net' is the wordplay.
'old thailand' becomes 'siam' (I've seen this before).
'briefly' means to remove the last letter.
'trapped in' means one lot of letters goes inside another.
'net' becomes 'mesh' ('mesh' can be a synonym of 'net').
'siam' with its final letter taken away is 'sia'.
'sia' put into 'mesh' is 'MESSIAH'.
'of' is the link.
(Other definitions for messiah that I've seen before include "Expected deliverer of Jewish people in Old Testament" , "His coming was prophesied in the Old Testament" , "Expected deliverer of the Jews" , "Handel's composition sung at Christmas" , "Handel's composition, sung annually by Our Lady's Choral Society" .)