Leapt from back of bus before crash (6)
I believe the answer is:
sprang
'leapt' is the definition.
(springing is a kind of leaping)
'back of bus before crash' is the wordplay.
'back of' says to take the final letters.
'before' means one lot of letters go next to another.
'crash' becomes 'prang' (to prang is to crash a car).
The final letter of 'bus' is 's'.
's'+'prang'='SPRANG'
'from' acts as a link.
(Other definitions for sprang that I've seen before include "Bounded forward" , "Jumped or leaped in single movement (6(" , "released from prison" , "freed prisoner" , "bounced" .)