Harm barrier with time (6)
I believe the answer is:
damage
'harm' is the definition.
(I know that harm can be written as damage)
'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' (an age is a period of time).
'dam'+'age'='DAMAGE'
(Other definitions for damage that I've seen before include "Mar" , "vandalism?" , "Harm, impair" , "destruction" , "Cost (slang)" .)