Mail is in each new carriage (4,6)
I believe the answer is:
post chaise
'carriage' is the definition.
(post chaise is a kind of carriage)
'mail is in each new' is the wordplay.
'mail' becomes 'post' (synonyms).
'in' is an insertion indicator.
'new' indicates an anagram.
'each' with letters rearranged gives 'chae'.
'is' inserted inside 'chae' is 'chaise'.
'post'+'chaise'='POST-CHAISE'
(Other definitions for post chaise that I've seen before include "carry letters in one" , "In old coach" , "Horse-drawn carriage" .)