Damaging (7)
'damaging' is the definition.
(impairing is a kind of damaging)
This is all the clue.
(Another definition for impairing that I've seen is " This is not helping".)
'damaging' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for harmful that I've seen before include "Injurious" , "Deleterious" , "Causing injury or damage, say poison or malice" , "Noxious" , "Half rum (anag.)" .)