Daring Nazis are after fortune hidden in tree (10)
I believe the answer is:
pluckiness
'daring' is the definition.
(I know that daring can be written as pluckiness)
'nazis are after fortune hidden in tree' is the wordplay.
'nazis' becomes 'SS' (the SS was a Nazi organisation).
'are after' says to put letters next to each other.
'fortune' becomes 'luck' (synonyms).
'hidden in' is an insertion indicator.
'tree' becomes 'pine' (pine is a kind of tree).
'luck' put inside 'pine' is 'pluckine'.
'ss' after 'pluckine' is 'PLUCKINESS'.
(Another definition for pluckiness that I've seen is " Courage".)