Mail is in each new carriage (4,6)
I believe the answer is:
post chaise
'carriage' is the definition.
(I know that post chaise is a type of carriage)
'mail is in each new' is the wordplay.
'mail' becomes 'post' (synonyms).
'in' indicates putting letters inside.
'new' is an anagram indicator.
'each' is an anagram of 'chae'.
'is' inserted within '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" .)