Number on University Challenge team notice one's slept in (4-6)
I believe the answer is:
four-poster
'one's slept in' is the definition.
(type of bed)
'number on university challenge team notice' is the wordplay.
'number on university challenge team' becomes 'four' (reference to quiz show University Challenge).
'notice' becomes 'poster' ('poster' can be a synonym of 'notice').
'four'+'poster'='FOUR-POSTER'
(Other definitions for four-poster that I've seen before include "Canopied bed" , "Large bed" , "Style of bed" , "Curtained bed" , "Type of bed" .)