Electric current, in various places, brings about wonders on screen (7,7)
'wonders on screen' is the definition.
'electric current in various places brings about' is the subsidiary indication.
'electric current' becomes 'i' (physics symbol).
'in' is an insertion indicator.
'various' indicates anagramming the letters.
'brings about' becomes 'effects' ('effect' can be a synonym of 'bring about'**).
'places' anagrammed gives 'specal'.
'i' placed within 'specal' is 'special'.
(Other definitions for special effects that I've seen before include "magic of the cinema!", "things put together after shooting", "tricks used in picture-making".)