One vehicle of ten, seemingly, in a plan (7)
I believe the answer is:
chariot
'one vehicle' is the definition.
(I know that chariot is a vehicle)
'ten seemingly in a plan' is the wordplay.
'ten seemingly' becomes 'io' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'a plan' becomes 'chart' (chart can mean a plan or diagram).
'io' put within 'chart' is 'CHARIOT'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for chariot that I've seen before include "Roman two-wheeled vehicle" , "ceremonial carriage" , "coach" , "four-wheeled carriage?" , "ride in carriage" .)