Exposed area always reflected light source (8)
I believe the answer is:
revealed
'exposed' is the definition.
('reveal' can be a synonym of 'expose')
'area always reflected light source' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'reflected' says the letters should be written backwards.
'light source' becomes 'led' (abbreviation for Light Emitting Diode).
'a'+'ever'='aever'
'aever' in reverse letter order is 'revea'.
'revea'+'led'='REVEALED'
(Other definitions for revealed that I've seen before include "Exposed" , "Made clear and visible" , "Like the religion" , "Shown" , "'Made known, disclosed (8)'" .)