Harm barrier with silver emulsion, initially (6)
I believe the answer is:
damage
'harm' is the definition.
(I know that harm can be written as damage)
'barrier with silver emulsion initially' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'with' means one lot of letters go next to another.
'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)'" .)