Call for sent mail to be unopened (6)
I believe the answer is:
entail
'call for' is the definition.
(entail can mean to call for or require)
'sent mail to be unopened' is the wordplay.
'sent' becomes 'ent' (I can't explain this - if you can you should believe this answer much more).
'to be unopened' means to remove the first letter (the opening of the word is removed).
'mail' with its initial letter removed is 'ail'.
'ent'+'ail'='ENTAIL'
Can you help me to learn more?
(Other definitions for entail that I've seen before include "Call for" , "outmoded fashion" , "Invlove or necessitate" , "settlement of estate" , "Inalienable estate" .)