A French flat one's in for free (6)
I believe the answer is:
unpaid
'free' is the definition.
('unpaid' can be similar in meaning to 'free')
'a french flat one's in' is the wordplay.
'a french' becomes 'un' ('a' in French).
'flat' becomes 'pad' (person's home).
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'pad' placed around 'i' is 'paid'.
'un'+'paid'='UNPAID'
'for' acts as a link.
(Other definitions for unpaid that I've seen before include "Of bill, outstanding" , "Without wages" , "Still owing" , "Not yet remunerated" , "Outstanding, owing" .)