At the outset so confident, relying on one’s generous ethos — not this guy’s! (7)
I believe the answer is:
scrooge
'not this guy's' is the definition.
The definition suggests a singular noun which matches the answer.
'at the outset so confident relying on one's generous ethos' is the wordplay.
'at the outset' says to take the initial letters (the outset is the beginning).
The first letters of 'so confident relying on ones generous ethos' is 'SCROOGE'.
(Other definitions for scrooge that I've seen before include "'A Christmas Carol' character" , "Miserly curmudgeon" , "He saw Marley's ghost" , "A Christmas Carol miser" , "Dickens' skinflint (7)" .)