Twelve, not a soul short (4)
I believe the answer is:
noon
'twelve' is the definition.
(I've seen this before)
'not a soul short' is the wordplay.
'not a soul' becomes 'noone' (I've seen this before).
'short' means to remove the last letter.
'noone' with its last letter removed is 'NOON'.
(Other definitions for noon that I've seen before include "Twelve during daylight" , "The middle of the day" , "A moment of time" , "when PM starts" , "Twelve o'clock, midday" .)