The sovereign King Charles's head is cut off during cruel revolution (5)
I believe the answer is:
ruler
'the sovereign' is the definition.
(I've seen this in another clue)
'king charles's head is cut off during cruel revolution' is the wordplay.
'king charles' becomes 'r' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'head is cut off during' means to remove the first letter.
'revolution' indicates an anagram.
'cruel' with its first letter taken off is 'ruel'.
'ruel' anagrammed gives 'uler'.
'r'+'uler'='RULER'
(Other definitions for ruler that I've seen before include "Aid for a straight line" , "It helps to draw straight lines" , "liner?" , "Straight edge; governor" , "one going straight?" .)