How one regretted making the acrimonious remark? (8)
I believe the answer is:
bitterly
'how one regretted' is the definition.
I can't judge whether this definition defines the answer.
'the acrimonious remark?' is the wordplay.
'the acrimonious' becomes 'bitter' (I've seen this before).
'remark?' becomes 'ly' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bitter'+'ly'='BITTERLY'
'making' is the link.
Can you help me to learn more?
(Other definitions for bitterly that I've seen before include "Sharply, pungently" , "Resentfully, rancorously" , "Acrimoniously" , "In a resentful manner" , "With resentment" .)