Coach one boards after pursuit (4,6)
I believe the answer is:
post chaise
'coach' is the definition.
(I have seen 'old coach' mean 'post chaise' so perhaps 'coach' could also mean 'post chaise')
'one boards after pursuit' is the wordplay.
'one' becomes 'i' (Roman numeral).
'boards' is an insertion indicator.
'after' becomes 'post' (I've seen this before).
'pursuit' becomes 'chase' (synonyms).
'post'+'chase'='postchase'
'i' inserted into 'postchase' is 'POST-CHAISE'.
(Other definitions for post chaise that I've seen before include "Horse-drawn carriage" , "In old coach" , "carry letters in one" .)