Pen and fold in an envelope (9)
I believe the answer is:
enclosure
This clue could be a double definition.
'pen' is the first definition.
(I've seen this before)
'fold in an envelope' is the second definition.
'enclosure' can be an answer for 'fold' (thesaurus). I'm unsure of the 'in an envelope' bit.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for enclosure that I've seen before include "One's contained" , "Area fenced off - one's ulcer (anag)" , "something to accompany letter?" , "pound" , "What's in the envelope" .)