Those on the bus when one sets out? (5)
I believe the answer is:
seats
'those on the bus' is the definition.
I can't tell whether this definition defines the answer.
'one sets out?' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'out?' indicates putting letters inside.
'a' put within 'sets' is 'SEATS'.
'when' is the link.
Can you help me to learn more?
(Other definitions for seats that I've seen before include "Chairs, stools" , "Parliamentary constituencies" .)