Crossword Genius

A merchant in Iran develops deepest depression (7,6)

Ross

I believe the answer is:

mariana trench

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

'a merchant' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'iran develops deepest depression' is the wordplay.
I cannot really understand how this works, but
an anagram of 'iran' is 'rian' which is within the answer.
'depression' could be 'trench' (trench is a kind of depression) and 'trench' is located in the answer.
The remaining letters 'maa' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'in' is the link.

Can you help me to learn more?

I've seen this clue in The Irish Times.
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