Principle involving guys in flat (8)
I believe the answer is:
tenement
'flat' is the definition.
(thesaurus)
'principle involving guys' is the wordplay.
'principle' becomes 'tenet' (I've seen this before).
'involving' is an insertion indicator.
'guys' becomes 'men'.
'tenet' enclosing 'men' is 'TENEMENT'.
'in' acts as a link.
(Other definitions for tenement that I've seen before include "Multiple-occupancy building" , "Run-down apartment house" , "Permanent property held by tenant" , "(Scottish) building comprising separate flats" , "Self-contained room or set of rooms" .)