At the outset, naturally - exactly when you entertain ambitious resolutions (3,4)
I believe the answer is:
new year
'at the outset naturally exactly when you entertain ambitious resolutions' is the definition.
I can't judge whether this definition defines the answer.
'at the outset naturally exactly when you entertain ambitious resolutions' is the wordplay.
'at the outset' suggests taking the first letters (the outset is the beginning).
The initial letters of 'naturally exactly when you entertain ambitious resolutions' is 'NEW YEAR'.
Can you help me to learn more?
(Other definitions for new year that I've seen before include "Time for first-footing" , "This is only five days old" , "Near yew (anag)" , "Annual starting point" , "different diary now?" .)