The most common and most likely answer to this crossword clue is the 7 letter word PILLION
I've seen this crossword clue in The Daily Telegraph
P
I
L
L
I
O
N
More clues leading to the result PILLION
- Hero on edge, turning over seat behind
- Contraceptive best on back seat
- Flipping cheek – Leo’s getting seat next to driver!
- Back seat taken by passengers
- Seat very good to get on, but with bad stuffing
- Saddle hurtful with one aboard little horse endlessly
- The back seat for one working in support of 12
- Back seat taken by individual working to support contraceptive
- The best learner sits at the edge, facing backwards to backseat
- Nerve required to climb over brave person in passenger seat
Buddy explains!

The straight part
We know that "The back seat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the back seat = pillion.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PILLION mean?
PILLION noun- A pad behind the saddle of a horse for a second rider.
- A similar second saddle on a motorcycle for a passenger.
- The person riding in the pillion.
- The cushion of a saddle.
- To ride on a pillion.
- To put a pillion on a horse.
- Riding behind the driving rider, as when positioned on the rump of a mount.