Stationery item for a sovereign (5)
I believe the answer is:
ruler
This clue is a double definition.
'stationery item' is the first definition.
(I've seen this before)
'sovereign' is the second definition.
(I've seen this in another clue)
'for a' is the link.
(Other definitions for ruler that I've seen before include "measure applied in school?" , "Emperor - measure" , "Straight edge; governor" , "that should go straight" , "Emperor, maybe" .)