Large insect regularly nearing light (7)
I believe the answer is:
lantern
'light' is the definition.
(lantern is a type of light)
'large insect regularly nearing' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'insect' becomes 'ant' (ant is a kind of insect).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'nearing' are 'ern'.
'l'+'ant'+'ern'='LANTERN'
(Other definitions for lantern that I've seen before include "It illuminates" , "outside D [DECORATION]" , "Lamp with protective transparent casing" , "Encased light" , "Portable light source" .)