Attacks with a 'furious fastidiousness', ultimately — fine to ignore flaws (8)
I believe the answer is:
Here is my best explanation:
'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'.
'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?