Drinks late on? They won't help you keep a cool head then (9)
I believe the answer is:
nightcaps
I believe this clue is a double definition.
'drinks late on?' is the first definition.
(drinks consumed very late on in the day)
'they won't help you keep a cool head then' is the second definition.
(a nightcap worn on the head might keep it warm)
(Other definitions for nightcaps that I've seen before include "Final round, perhaps" , "assumed to prevent head colds" , "Drinks at bedtime" , "Late drinks" .)