Union preventing monarch leaving (4)
I believe the answer is:
bloc
'union' is the definition.
(I've seen this in another clue)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for bloc that I've seen before include "Group of countries in special alliance" , "Political group of nations" , "Group with a common policy" , "Alliance - coalition" , "Political coalition" .)