This person's two ducks causing damage (6)
I believe the answer is:
impair
'damage' is the definition.
(impairing is a kind of damaging)
'this person's two' is the wordplay.
'this person's' becomes 'im' (this person means the crossword setter themselves).
'two' becomes 'pair' (pair is a kind of two).
'im'+'pair'='IMPAIR'
'ducks causing' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for impair that I've seen before include "Hurt or handicap" , "Reduce" , "Damage or handicap" , "Hinder" , "Damage or injure" .)