Edict protecting a source of silicon means reduction (8)
I believe the answer is:
decrease
'reduction' is the definition.
('decrease' can be a synonym of 'reduction')
'edict protecting a source of silicon' is the wordplay.
'edict' becomes 'decree' ('decree' can be a synonym of 'edict'**).
'protecting' means one lot of letters goes inside another.
'source of' says to take the initial letters.
The initial letter of 'silicon' is 's'.
'a'+'s'='as'
'decree' going around 'as' is 'DECREASE'.
'means' is the link.
(Other definitions for decrease that I've seen before include "cut back" , "Contract" , "Dock" , "Make or become less" , "Reduction" .)