Party with rector during the hours of darkness? Absolutely! (9)
I believe the answer is:
Here is my best explanation:
'absolutely' is the definition.
'party with rector during the hours of darkness?' is the subsidiary indication.
'party' becomes 'do' (do is a kind of party).
'with' becomes 'w' (abbreviation).
'rector' becomes 'r'.
'during' means one lot of letters goes inside another.
'the hours of darkness?' becomes 'night' (I've seen this before).
'r' placed within 'night' is 'nright'.
(Other definitions for downright that I've seen before include "Completely", "Frank", "Utter, arrant", "blunt", "Complete", "Utter, complete", "Out-and-out", "True", "Plain-spoken" and "Absolutely!".)