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' is a charade indicator (letters next to each other).
'run' becomes 'dash' (dashing is a kind of running).
'slap'+'dash'='SLAPDASH'
(Other definitions for slapdash that I've seen before include "Cowboy" , "recklessly" , "In a careless manner" , "disorganized" , "Careless - slack" .)