Backing account, criticise one in remark (13)
I believe the answer is:
accompaniment
'backing' is the definition.
(I've seen this before)
'account criticise one in remark' is the wordplay.
'account' becomes 'ac' (abbreviation).
'criticise' becomes 'pan' (informal term meaning to criticise heavily).
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'remark' becomes 'comment' (commenting is a kind of remarking).
'pan'+'i'='pani'
'pani' inserted into 'comment' is 'companiment'.
'ac'+'companiment'='ACCOMPANIMENT'
(Other definitions for accompaniment that I've seen before include "Supporting musical line" , "Musical support?" , "Complement" , "Music supporting a singer" , "Backing" .)