Enclosed duplicated letters in deep mystery (6)

I believe the answer is:

penned

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

'enclosed' is the definition.
(similar in meaning)

'duplicated letters in deep mystery' is the wordplay.
'duplicated letters' becomes 'nn' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'mystery' indicates an anagram.
'deep' with letters rearranged gives 'peed'.
'nn' placed into 'peed' is 'PENNED'.

Can you help me to learn more?

(Other definitions for penned that I've seen before include "Confined; written" , "Caged, wrote" , "in Reading Gaol, perhaps" , "Corralled" , "Shut in" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.