Organ originally borne on large farm cart (7)
I believe the answer is:
tumbrel
'farm cart' is the definition.
'organ originally borne on large' is the wordplay.
'organ' becomes 'tum' (tum is a kind of organ).
'originally' indicates taking the first letters.
'on large' becomes 'rel' (I can't explain this - if you can you should believe this answer much more).
The first letter of 'borne' is 'b'.
'tum'+'b'+'rel'='TUMBREL'
(Other definitions for tumbrel that I've seen before include "transport to block" , "prisoner transport" , "Cart in which aristocrats were taken to guillotine" , "Cart in which aristocrats went to the guillotine" , "Open cart" .)