Old coffer in studies becomes mysterious (7)

I believe the answer is:

darkens

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'becomes mysterious' is the definition.
I can't judge whether this definition defines the answer.

'old coffer in studies' is the wordplay.
'old coffer' becomes 'ark' (I've seen this before. I am not sure about the 'old' bit.).
'in' means one lot of letters goes inside another.
'studies' becomes 'dens' (I've seen this in other clues).
'ark' inserted into 'dens' is 'DARKENS'.

Can you help me to learn more?

(Other definitions for darkens that I've seen before include "becomes gloomy" , "Gets black" .)

I've seen this clue in The Independent.

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