Harm barrier, with time (6)
'harm' is the definition.
'barrier with time' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'with' says to put letters next to each other.
'time' becomes 'age' (age is a kind of time).
(Other definitions for damage that I've seen before include "Mar", "vandalism?", "Harm, impair", "destruction", "Cost (slang)".)