The first to knock carriage (4)
I believe the answer is:
tram
'knock carriage' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'the first' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'first' becomes 'ram' (I can't justify this - if you can you should believe this answer much more).
't'+'ram'='TRAM'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for tram that I've seen before include "Electrically driven transport" , "Road vehicle running on tracks" , "Public transport vehicle" , "Passenger vehicle on rails" , "Former public vehicle" .)