Brush aside single stick that knight splits (6)
I believe the answer is:
ignore
'brush aside' is the definition.
(synonyms)
'single stick that knight splits' is the wordplay.
'single' becomes 'I' (one).
'stick' becomes 'gore' (both can mean to pierce or skewer something).
'that knight' becomes 'N' (chess abbreviation).
'splits' indicates putting letters inside (inserted letters split open the word).
'gore' enclosing 'n' is 'gnore'.
'i'+'gnore'='IGNORE'
(Other definitions for ignore that I've seen before include "'Disregard, overlook (6)'" , "Fail to consider" , "Pay no attention to, disregard" , "Pass over" , "Intentionally disregard" .)