Payment I posted in error (7)
I believe the answer is:
deposit
'payment' is the definition.
(deposit is a kind of payment)
'i posted in error' is the wordplay.
'in error' indicates anagramming the letters.
'i'+'posted'='iposted'
'iposted' is an anagram of 'DEPOSIT'.
(Other definitions for deposit that I've seen before include "Plant" , "First instalment" , "Part payment on a purchase" , "Money in bank - part payment" , "Precipitate" .)