Blemish in crown of Victorian diamonds (4)
I believe the answer is:
vice
'blemish' is the definition.
(thesaurus)
'crown of victorian diamonds' is the wordplay.
'crown of' says to take the initial letters.
'diamonds' becomes 'ice' (I've seen this before).
The first letter of 'victorian' is 'v'.
'v'+'ice'='VICE'
'in' acts as a link.
(Other definitions for vice that I've seen before include "Prefix meaning acting as deputy" , "Wrong" , "Is the gripping tool evil?" , "Prefix for deputy to, say, chairman" , "Gripper - in place of - immorality" .)