Harm barrier with silver emulsion, initially (6)
I believe the answer is:
damage
'harm' is the definition.
('damage' can be a synonym of 'harm')
'barrier with silver emulsion initially' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier**).
'with' says to put letters next to each other.
'silver' becomes 'ag' (Ag is the chemical symbol for silver).
'initially' says to take the initial letters.
The first letter of 'emulsion' is 'e'.
'dam'+'ag'+'e'='DAMAGE'
(Other definitions for damage that I've seen before include "Harm, loss" , "To vandalise" , "Injure, spoil" , "Detrimental effect" , "'Deface, impair (6)'" .)