Outgoing mail, I assume (5)
I believe the answer is:
posit
'assume' is the definition.
(I know that posit is a type of presuppose)
'outgoing mail i' is the wordplay.
'outgoing' is an insertion indicator.
'mail' becomes 'post' (synonyms).
'post' placed around 'i' is 'POSIT'.
(Other definitions for posit that I've seen before include "Put forward; place" , "Put forward as true" , "Assume as a fact or principle" , "Assume something as a basis for discussion" , "Propose" .)