Endure changes about this time, not having an agent (13)

I believe the answer is:

unrepresented

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'having an agent' is the definition.
I can't judge whether this defines the answer.

'endure changes about this time not' is the wordplay.
'changes' is an anagram indicator.
'about' is an insertion indicator.
'this time not' becomes 'present' (I can't explain this - if you can you should give a lot more credence to this answer).
'endure' is an anagram of 'unreed'.
'unreed' going around 'present' is 'UNREPRESENTED'.

Can you help me to learn more?

(Another definition for unrepresented that I've seen is " without agent".)

I've seen this clue in The Irish Independent.

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