Sort of letter to take out of the envelope (4)
I believe the answer is:
open
This clue might be a double definition.
'sort of letter' is the first definition.
'out of the envelope' is the second definition.
'open' can be an answer for 'out' (thesaurus). I am unsure of the 'of the envelope' bit.
'to take' acts as a link.
Can you help me to learn more?
(Other definitions for open that I've seen before include "Not closed; set going" , "Admitting customers" , "Start - frank" , "Candid and undisguised" , "yawning" .)