A little spirit, Nat's nightcap, served up without delay (7)
I believe the answer is:
instant
'without delay' is the definition.
(instant means at once or without delay)
'a little spirit nat's nightcap served up' is the wordplay.
'a little' indicates a hidden word (a little bit of the clue).
'served up' shows that the letters should be reversed in order (in a down clue, going upwards).
'tnatsni' is hidden in the letters of 'spirit nats nightcap'.
'tnatsni' in reverse letter order is 'INSTANT'.
(Other definitions for instant that I've seen before include "pressing" , "This coffee is made from powder" , "Trice" , "Jiffy" , "A second - coffee, maybe" .)