Sovereign stationery item (5)
I believe the answer is:
ruler
I believe this is a double definition.
'sovereign' is the first definition.
(I've seen this before)
'stationery item' is the second definition.
(I've seen this before)
(Other definitions for ruler that I've seen before include "Source of line" , "Queen, say" , "Leader - line-drawing aid" , "Thoroughly straight" , "Strip used in drawing straight lines" .)