Not living together, guys before beginning to tenant flat (9)
I believe the answer is:
apartment
'flat' is the definition.
('apartment' can be a synonym of 'flat')
'not living together guys before beginning to tenant' is the wordplay.
'not living together' becomes 'apart' (I've seen this before).
'guys' becomes 'men'.
'before' means one lot of letters go next to another.
'beginning to' suggests taking the first letters.
The initial letter of 'tenant' is 't'.
'apart'+'men'+'t'='APARTMENT'
(Other definitions for apartment that I've seen before include "quarters" , "pad" , "place to live" , "Flat (chiefly US)" , "19 across" .)