Black stuff covering small ruler once (4)
I believe the answer is:
'ruler once' is the definition.
I can't judge whether this defines the answer.
'black stuff covering small' is the wordplay.
'black stuff' becomes 'tar' (tar is a black substance).
'covering' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tar' enclosing 's' is 'TSAR'.
Can you help me to learn more?
(Other definitions for tsar that I've seen before include "Russian leader" , "Supreme authority" , "Eg, Ivan the Terrible" , "Old Russian 22" , "Tyrant" .)