Touch upon sum overdue and outstanding (6)
I believe the answer is:
allude
'touch' is the definition.
(I know that touch can be written as allude)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for allude that I've seen before include "Refer to, speak of" , "Speak about in passing" , "Refer (to) in passing" , "give a reference" , "Refer briefly or indirectly in speaking" .)