Rascal's discourteous then nice, not one to show lack of caution (10)
I believe the answer is:
imprudence
'lack of caution' is the definition.
(I've seen this before)
'rascal's discourteous then nice not one' is the wordplay.
'rascal' becomes 'imp' ('imp' can be a synonym of 'rascal'**).
'discourteous' becomes 'rude' (I've seen this before).
'then' is a charade indicator (letters next to each other).
'not' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'nice' with 'i' removed is 'nce'.
'imp'+'rude'+'nce'='IMPRUDENCE'
'to show' is the link.
(Other definitions for imprudence that I've seen before include "Folly" , "foolishness" , "An unwise act" , "Injudicious act" , "lack of caution" .)