Number of years below average adult spent in household (6)
I believe the answer is:
menage
'household' is the definition.
('menage' can be a synonym of 'household')
'number of years below average adult spent in' is the wordplay.
'number of years' becomes 'age'.
'below' means one lot of letters go next to another.
'average' becomes 'mean' (type of average).
'adult' becomes 'a'.
'spent in' indicates named letters should be taken away (to spend can mean to waste or give away).
'mean' with 'a' removed is 'men'.
'age' after 'men' is 'MENAGE'.
(Other definitions for menage that I've seen before include "live together" , "Social unit living together" , "A household, sometimes naughtily a trois" , "Persons of household" , "a household, may be naughtily a trois" .)