Black stuff covering small ruler once (4)

I believe the answer is:

tsar

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'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" .)

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