Trips and falls under the back of the bus (8)
I believe the answer is:
stumbles
'trips' is the definition.
(I know this)
'falls under the back of the bus' is the wordplay.
'falls' becomes 'tumbles' (tumbling is a kind of falling).
'under' means one lot of letters go next to another (below, in a down clue).
'the back of the bus' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'tumbles' put after 's' is 'STUMBLES'.
'and' is the link.
Can you help me to learn more?
(Other definitions for stumbles that I've seen before include "Comes near to falling" , "Strikes foot on something and loses balance" , "Trips and almost falls" .)