Forty winks? Just under twelve (4)
I believe the answer is:
Here is my best explanation:
'forty winks?' is the definition.
'just under twelve' is the wordplay.
'just under' means to remove the last letter (the majority but not all of the word is written).
'twelve' becomes 'dozen' ('dozen' can be a synonym of 'twelve').
'dozen' with its final letter taken away is 'DOZE'.
(Other definitions for doze that I've seen before include "Nod off", "Nap", "Sleep fitfully", "Sleep lightly", "Catnap", "Light sleep", "A short light sleep", "Take nap", "Go to sleep" and "Have a nap".)