Sovereign's representative (7)
I believe the answer is:
'sovereign's representative' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for viceroy that I've seen before include "Ruler of a colony" , "Monarch's plenipotentiary - American butterfly, like (but smaller than) a monarch" , "One ruling in a colony" , "sovereign's governor" , "Sovereign's representative in a colony" .)