Cretins with no idea turning rude (13)
I believe the answer is:
inconsiderate
'rude' is the definition.
(thesaurus)
'cretins with no idea turning' is the wordplay.
'with' means one lot of letters go next to another.
'turning' indicates anagramming the letters.
'no'+'idea'='noidea'
'cretins' put after 'noidea' is 'noideacretins'.
'noideacretins' with letters rearranged gives 'INCONSIDERATE'.
(Other definitions for inconsiderate that I've seen before include "intolerant" , "Lacking regard for the feelings of others" , "Selfish" , "Thoughtlessly hurtful" , "Thoughtless of others" .)