Careless mates upset by crash (8)
I believe the answer is:
slapdash
'careless' is the definition.
(similar in meaning)
'mates upset by crash' is the wordplay.
'mates' becomes 'pals'.
'upset' shows that the letters should be reversed in order.
'by crash' becomes 'dash' (I can't explain this - if you can you should give a lot more credence to this answer).
'pals' back-to-front is 'slap'.
'slap'+'dash'='SLAPDASH'
Can you help me to learn more?
(Other definitions for slapdash that I've seen before include "Careless - slack" , "disorganised" , "Hasty and careless - hit and run?" , "Perfunctory" , "Careless - haphazard" .)