Her tract upset one of four joint rulers (8)

I believe the answer is:

tetrarch

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'one of four joint rulers' is the definition.
The definition suggests a singular noun which matches the answer.

'her tract upset' is the wordplay.
'upset' indicates an anagram.
'her'+'tract'='hertract'
'hertract' with letters rearranged gives 'TETRARCH'.

Can you help me to learn more?

(Other definitions for tetrarch that I've seen before include "Joint leader" , "Herod perhaps" , "One of four rulers" .)

I've seen this clue in The Independent.

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