Plain English, extremely light (8)
I believe the answer is:
everyday
'plain' is the definition.
(thesaurus)
'english extremely light' is the wordplay.
'english' becomes 'E' (abbreviation as in OED).
'extremely' becomes 'very' (very can mean extremely or considerably).
'light' becomes 'day' (I've seen this in other clues).
'e'+'very'+'day'='EVERYDAY'
(Other definitions for everyday that I've seen before include "Commonplace and ordinary" , "familiar" , "Run-of-the-mill" , "Happening seven times a week" , "Mundane, ordinary" .)