A sovereign for a stationery item (5)
I believe the answer is:
ruler
This clue is a double definition.
'a sovereign' is the first definition.
(I've seen this before)
'stationery item' is the second definition.
(I've seen this before)
'for a' acts as a link.
(Other definitions for ruler that I've seen before include "One who commands - often seen in the classroom" , "Measuring tool" , "person holding sway?" , "Person in authority" , "Strip used in drawing straight lines" .)