Briskness, promptness (8)
I believe the answer is:
alacrity
I believe this clue is a double definition.
'briskness' is the first definition.
(I know that briskness can be written as alacrity)
'promptness' is the second definition.
(I've seen this before)
(Other definitions for alacrity that I've seen before include "Showing brisk readiness" , "'Briskness, promptness (8)'" , "Quickness" , "Brisk and cheerful readiness" , "Lively enthusiasm" .)