Proper masculine domain (5)
I believe the answer is:
realm
'domain' is the definition.
(I've seen this before)
'proper masculine' is the wordplay.
'proper' becomes 'real' ('real' can be similar in meaning to 'proper').
'masculine' becomes 'm'.
'real'+'m'='REALM'
(Other definitions for realm that I've seen before include "Kingdom; field of interest" , "Royal domain" , "Area of knowledge under discussion" , "A Nation" , "Lamer (anag.)" .)