Harm barrier, over many years (6)
I believe the answer is:
damage
'harm' is the definition.
(synonyms)
'barrier over many years' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'over' is a charade indicator (letters next to each other).
'many years' becomes 'age' (synonyms. I am not sure about the 'many' bit.).
'dam'+'age'='DAMAGE'
Can you help me to learn more?
(Other definitions for damage that I've seen before include "Deface, break" , "Impairment, injury" , "vandalism?" , "Ravages" , "Impair and lower value" .)