Outstanding secretary one had, inferior to Pierre's one (6)
I believe the answer is:
unpaid
'outstanding' is the definition.
(unpaid debts are still outstanding)
'secretary one had inferior to pierre's one' is the wordplay.
'secretary' becomes 'PA' (abbreviation for personal assistant).
'one had' becomes 'Id' ('I'd' can mean 'one had').
'inferior to' says to put letters next to each other (in a down clue, some letters go below others).
'pierre's one' becomes 'un' (I am not sure about this - if you are sure you should believe this answer much more).
'pa'+'id'='paid'
'paid' put after 'un' is 'UNPAID'.
Can you help me to learn more?
(Other definitions for unpaid that I've seen before include "(Of debt) outstanding" , "Not yet remunerated" , "Without wages" , "Overdue" , "Of bill, outstanding" .)