Crossword Genius

An old ruler (4)

Ross

I believe the answer is:

tsar

I'm a little stuck... Click here to teach me more about this clue!

'an old ruler' is the definition.
(I've seen this in another clue)

'an old ruler' is the wordplay.
I cannot really see how this works, but
'an' could be 'a' and 'a' is present in the answer.
'ruler' could be 'r' (abbreviation for rex, king in Latin) and 'r' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for tsar that I've seen before include "Nicholas II, say" , "Title given to the kings of Bulgaria" , "Alexander and Nicholas held this title in Russia" , "Autocrat" , "Person in charge" .)

I've seen this clue in the Evening Standard.
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