Outstanding day after first of October (4)
I believe the answer is:
owed
'outstanding' is the definition.
(as in an outstanding debt)
'day after first of october' is the wordplay.
'day' becomes 'wed' (abbreviation for Wednesday).
'after' says to put letters next to each other.
'first of october' becomes 'o' (1st letter of 'october').
'wed' after 'o' is 'OWED'.
(Other definitions for owed that I've seen before include "(Sum) due for payment" , "Was due to pay" , "Outstanding" , "Was indebted" , "to be paid" .)