Stout friend, about 50, one sharing digs (8)
I believe the answer is:
flatmate
'one sharing digs' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'stout friend about 50' is the wordplay.
'stout' becomes 'fat' (similar in meaning).
'friend' becomes 'mate' (mate is a kind of friend).
'about' indicates putting letters inside.
'50' becomes 'l' (Roman numeral).
'fat'+'mate'='fatmate'
'fatmate' enclosing 'l' is 'FLATMATE'.
Can you help me to learn more?
(Other definitions for flatmate that I've seen before include "I may share with someone" , "Person sharing apartment" , "one sharing same pad" , "Sharer of accommodation" , "Apartment sharer" .)