So cheap it's ridiculous, this previous form of transport (4,6)
I believe the answer is:
post chaise
'transport' is the definition.
(post chaise is a kind of transport)
'so cheap it's ridiculous' is the wordplay.
'ridiculous' indicates an anagram.
'so'+'cheap'+'its'='socheapits'
'socheapits' is an anagram of 'POST-CHAISE'.
'this previous form of' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for post chaise that I've seen before include "In old coach" , "carry letters in one" , "Horse-drawn carriage" .)