Electric light circuit disrupted by introduction of mains (4)
I believe the answer is:
lamp
'electric light' is the definition.
(I know that lamp is a type of electric light)
'circuit disrupted by introduction of mains' is the wordplay.
'circuit' becomes 'lap' (synonyms).
'disrupted by' means one lot of letters goes inside another (inside letters 'disrupt' the flow of outside one).
'introduction of' says to take the initial letters.
The first letter of 'mains' is 'm'.
'lap' enclosing 'm' is 'LAMP'.
(Other definitions for lamp that I've seen before include "Device providing illumination" , "Florence Nightingale was the Lady with the . . . ." , "Artificial source of illumination" , "bulb" , "Appliance producing light" .)