Monarch crossing eastern border (3)
I believe the answer is:
hem
'border' is the definition.
(I've seen this before)
'monarch crossing eastern' is the wordplay.
'monarch' becomes 'hm' (short for Her Majesty).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'eastern' becomes 'e'.
'hm' enclosing 'e' is 'HEM'.
(Other definitions for hem that I've seen before include "Bottom of a garment" , "Finished edge of a piece of cloth" , "Express hesitation" , "Border; fringe" , "Double-backed cloth border" .)