Moderate payment in advance expected to arrive (6)
I believe the answer is:
subdue
'moderate' is the definition.
(subduing is a kind of moderating)
'payment in advance expected to arrive' is the wordplay.
'payment' becomes 'sub' (I have seen 'Advance payment ' mean 'sub' so perhaps 'payment' could also mean 'sub').
'in advance' is a charade indicator (letters next to each other).
'expected to arrive' becomes 'due' (I've seen this in another clue).
'sub'+'due'='SUBDUE'
(Other definitions for subdue that I've seen before include "Overcome, force to yield" , "Overcome, suppress" , "Overcome or inhibit" , "'Quieten, bring under control (6)'" , "Overpower and bring under control" .)