Principle covering boyfriends in flat (8)
I believe the answer is:
tenement
'flat' is the definition.
(type of apartment building)
'principle covering boyfriends' is the wordplay.
'principle' becomes 'tenet' (a tenet is a principle or value).
'covering' is an insertion indicator.
'boyfriends' becomes 'men' (man can mean someone's boyfriend).
'tenet' going around 'men' is 'TENEMENT'.
'in' is the link.
(Other definitions for tenement that I've seen before include "Run-down house for many people" , "Piece of owned land" , "Self-contained room or set of rooms" , "Building containing several dwellings" , "Rundown house with many occupants" .)