Write an enclosure (3)

I believe the answer is:

pen

I believe this is a double definition.

'write' is the first definition.
(I know that write can be written as pen)

'an enclosure' is the second definition.
(I know that pen is a type of enclosure)

(Other definitions for pen that I've seen before include "American jug?" , "Put down" , "Writing implement" , "International writers's organisation" , "Coop" .)

I've seen this clue in the Evening Standard.

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