Pen something to send with the letter (9)
I believe the answer is:
enclosure
'pen' is the definition.
(I've seen this in another clue)
'something to send with the letter' is the wordplay.
I cannot quite see how this works, but
'letter' could be 'en' (I've seen this in other clues) and 'en' is found in the answer.
The remaining letters 'closure' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for enclosure that I've seen before include "Sealed-off area" , "Pen" , "Document included with a letter" , "What's in the envelope" , "something to accompany letter?" .)