Nobleman, mostly bent, importing diamonds to European country (8)
I believe the answer is:
archduke
'nobleman' is the definition.
(I have seen 'Top nobleman' mean 'archduke' so perhaps 'nobleman' could also mean 'archduke')
'mostly bent importing diamonds to european country' is the wordplay.
'mostly' means to remove the last letter.
'bent' becomes 'arched' (arching is a kind of bending).
'importing' is an insertion indicator.
'diamonds' becomes 'd' (abbreviation in bridge say).
'to european country' becomes 'uk' (United Kingdom).
'arched' with its final letter removed is 'arche'.
'd'+'uk'='duk'
'arche' enclosing 'duk' is 'ARCHDUKE'.
(Other definitions for archduke that I've seen before include "European noble" , "Austrian noble" , "Title of Franz Ferdinand, shot in Sarajevo in 1914" , "Emperor's son" , "Prince of the former ruling house of Austria, Ferdinand for one" .)