Regretted introducing one point; caused irreparable damage (6)
I believe the answer is:
ruined
'caused irreparable damage' is the definition.
Although both the answer and definition are past participle verbs, I don't see how they can define each other.
'regretted introducing one point' is the wordplay.
'regretted' becomes 'rued' ('rue' can be a synonym of 'regret').
'introducing' means one lot of letters goes inside another (some letters introduced or inserted into others).
'one' becomes 'i' (Roman numeral).
'point' becomes 'n' (N is an example).
'i'+'n'='in'
'rued' going around 'in' is 'RUINED'.
Can you help me to learn more?
(Other definitions for ruined that I've seen before include "Reduced to poverty; damaged" , "Reduced to a state of poverty" , "Bankrup - destroyed" , "Spoilt" , "Bust" .)