Inaccurate Armenian chart: it's abysmal (7,6)

I believe the answer is:

mariana trench

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'abysmal' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'inaccurate armenian chart' is the wordplay.
'inaccurate' is an anagram indicator.
'armenian'+'chart'='armenianchart'
'armenianchart' anagrammed gives 'MARIANA TRENCH'.

'it's' acts as a link.

Can you help me to learn more?

I've seen this clue in The Independent.

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