Medication puts one on the back seat (7)

I believe the answer is:

pillion

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'back seat' is the definition.
(I've seen this before)

'medication puts one on' is the wordplay.
'medication puts' becomes 'pill' (I am not sure about this - if you are sure you should believe this answer much more).
'one' becomes 'i' (Roman numeral).
'pill'+'i'+'on'='PILLION'

'the' acts as a link.

Can you help me to learn more?

(Other definitions for pillion that I've seen before include "Seat behind a motorbike rider" , "Passenger seat at rear of motor-cycle" , "Back seat" , "Passenger seat at rear of motorbike" .)

I've seen this clue in the Evening Standard.

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