In accordance with boy holding one defamation (9)
I believe the answer is:
aspersion
'defamation' is the definition.
(synonyms)
'in accordance with boy holding one' is the wordplay.
'in accordance with' becomes 'asper' (I've seen this in another clue).
'boy' becomes 'Si'.
'holding one' becomes 'on' (I can't justify this - if you can you should believe this answer much more).
'asper'+'si'+'on'='ASPERSION'
Can you help me to learn more?
(Other definitions for aspersion that I've seen before include "No praises (anag) - disparaging remark" , "Attack on the character" , "Malicious remark" , "reflection" , "No praises given to the act of defaming 99)" .)