A back seat (7)
'a back seat' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pillion that I've seen before include "Back seat" , "Passenger seat at rear of motorbike" , "Passenger seat at rear of motor-cycle" , "Seat behind a motorbike rider" .)
This clue could be a double definition.
'a back' is the first 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?
'seat' is the second definition.
(I know that saddle is a type of seat)
Can you help me to learn more?
(Other definitions for saddle that I've seen before include "piece of lamb?" , "Seat for riding" , "Cut of meat" , "Equestrian seat" , "Cycle seat" .)