Careless hit-and-run (8)
I believe the answer is:
slapdash
'careless' is the definition.
(similar in meaning)
'hit-and-run' is the wordplay.
'hit' becomes 'slap' (both can mean to strike).
'and' says to put letters next to each other.
'run' becomes 'dash' (dash is a kind of run).
'slap'+'dash'='SLAPDASH'
(Other definitions for slapdash that I've seen before include "recklessly" , "Irresponsible" , "Careless - haphazard" , "Hastily and carelessly done" , "Slipshod, sloppy" .)