Something worth having, including new agreement (6)
I believe the answer is:
assent
'agreement' is the definition.
(I know that assent is a type of agreement)
'something worth having including new' is the wordplay.
'something worth having' becomes 'asset' (an asset is something valuable or useful).
'including' indicates putting letters inside.
'new' becomes 'n' (abbreviation as in New Testament NT).
'asset' enclosing 'n' is 'ASSENT'.
(Other definitions for assent that I've seen before include "Accede, agree" , "Agreement (to a proposal)" , "Acquiesce, comply" , "Acquiescence" , "Express approval or agreement" .)