Unit of measurement for each gold sovereign (7)
I believe the answer is:
emperor
'sovereign' is the definition.
(emperor is a type of sovereign ruler)
'unit of measurement for each gold' is the wordplay.
'unit of measurement' becomes 'em' (em is a kind of unit of measurement).
'for each' becomes 'per' (per person).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'em'+'per'+'or'='EMPEROR'
(Other definitions for emperor that I've seen before include "Kaiser" , "noted by Beethoven" , "Say, Nero" , "Purple --; -- penguin" , "Japanese version of the king" .)