Mediocre writer returned Japanese currency in carriage (7)
I believe the answer is:
hackney
'carriage' is the definition.
(I know that hackney is a type of carriage)
'mediocre writer returned japanese currency' is the wordplay.
'mediocre writer' becomes 'hack' (I've seen this before).
'returned' shows that the letters should be reversed in order.
'japanese currency' becomes 'yen' (the currency of Japan is the yen).
'yen' written backwards gives 'ney'.
'hack'+'ney'='HACKNEY'
'in' is the link.
(Other definitions for hackney that I've seen before include "A kind of taxi or car for hire" , "draught horse" , "Old cab - London borough" , "Wear out" , "Carriage for hire" .)