Plato upset about king's watch (6)
I believe the answer is:
patrol
'watch' is the definition.
(I know that patrol is a type of guard)
'plato upset about king's' is the wordplay.
'upset' indicates anagramming the letters.
'about' indicates putting letters inside.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'plato' is an anagram of 'patol'.
'patol' going around 'r' is 'PATROL'.
(Other definitions for patrol that I've seen before include "Guard's regular walk" , "Unit of a Boy Scout group" , "Reconnaissance unit" , "Move round and keep watch" , "Unit of Scout troop" .)