Careless man from Newcastle holds informal party (9)
I believe the answer is:
negligent
'careless' is the definition.
(similar in meaning)
'man from newcastle holds informal party' is the wordplay.
I cannot really see how this works, but
'man' could be 'gent' (gent is a kind of man) and 'gent' is located in the answer.
'from' could be 'e' (I've seen this in other clues) and 'e' is located in the answer.
'party' could be 'l' and 'l' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for negligent that I've seen before include "Lax" , "Careless, remiss" , "Failing to take proper care" , "sloppy" , "Not taking due care" .)