One visiting a French apartment, due (6)
I believe the answer is:
unpaid
'due' is the definition.
(I've seen this in another clue)
'one visiting a french apartment' is the wordplay.
'one' becomes 'i' (Roman numeral).
'visiting' indicates putting letters inside.
'a french' becomes 'un' ('a' in French).
'apartment' becomes 'pad' (I've seen this in another clue).
'un'+'pad'='unpad'
'i' placed inside 'unpad' is 'UNPAID'.
(Other definitions for unpaid that I've seen before include "(Of debt) outstanding" , "Of bill, outstanding" , "Overdue" , "Without reward" , "Honorary" .)