Letters and, in fact, mail regularly seen after delivery (9)
I believe the answer is:
postnatal
'after delivery' is the definition.
(I've seen this before)
'letters and in fact mail regularly' is the wordplay.
'letters' becomes 'post' (letters in the mail or post).
'and' is a charade indicator (letters next to each other).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'in'+'fact'+'mail'='infactmail'
The alternate letters of 'infactmail' are 'natal'.
'post'+'natal'='POSTNATAL'
'seen' acts as a link.
(Other definitions for postnatal that I've seen before include "In period after birth" , "a time of new life" , "Immediately after birth" , "after delivery" .)