Comfortable including new version of true politeness (8)
I believe the answer is:
courtesy
'politeness' is the definition.
(I know that courtesy is a type of politeness)
'comfortable including new version of true' is the wordplay.
'comfortable' becomes 'cosy' (similar in meaning).
'including' indicates putting letters inside.
'new version of' indicates anagramming the letters.
'true' is an anagram of 'urte'.
'cosy' enclosing 'urte' is 'COURTESY'.
(Other definitions for courtesy that I've seen before include "Respectful and considerate behaviour" , "Civility, politeness" , "Politeness and respect for others" , "Polite behaviour" , "Good manners" .)