Attacks with a 'furious fastidiousness', ultimately — fine to ignore flaws (8)
I believe the answer is:
assaults
'attacks' is the definition.
('assault' can be a synonym of 'attack')
'ultimately fine to ignore flaws' is the wordplay.
'ultimately' becomes 'ass' (I am not sure about this - if you are sure you should believe this answer much more).
'fine' becomes 'f' (abbreviation).
'to ignore' suggests deleting specific letters.
'flaws' becomes 'faults' ('fault' can be a synonym of 'flaw').
'faults' with 'f' removed is 'aults'.
'ass'+'aults'='ASSAULTS'
'with a furious fastidiousness' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for assaults that I've seen before include "Physical or emotional attacks" , "criminal acts" , "Violent or personal attacks" , "raids" , "'Attacks, violations (8)'" .)