A singular error masks good attribute (6)
I believe the answer is:
assign
'attribute' is the definition.
(synonyms)
'a singular error masks good' is the wordplay.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'error' becomes 'sin' (I've seen this before).
'masks' indicates putting letters inside (mask can mean to cover up or conceal).
'good' becomes 'g' (abbreviation).
'sin' placed around 'g' is 'sign'.
'a'+'s'+'sign'='ASSIGN'
(Other definitions for assign that I've seen before include "Allocate (a task or duty)" , "Transfer (legal rights)" , "Allot, allocate" , "Allot or ascribe" , "(Of a job) allocate to someone" .)