Flat principle about soldiers (8)
I believe the answer is:
tenement
'flat' is the definition.
(type of apartment building)
'principle about soldiers' is the wordplay.
'principle' becomes 'tenet' (a tenet is a principle or value).
'about' means one lot of letters goes inside another.
'soldiers' becomes 'men' (men can mean a group of soldiers).
'tenet' going around 'men' is 'TENEMENT'.
(Other definitions for tenement that I've seen before include "Scots maybe in this dwelling" , "Flat" , "Holding" , "Rented accommodation" , "Dwelling-place" .)