Long time left in darkness -- this will brighten things up (4,5)
I believe the answer is:
neon light
'will brighten things up' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'long time left in darkness' is the wordplay.
'long time' becomes 'eon' (eon is a kind of long time).
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'darkness' becomes 'night' (night is a period of darkness).
'eon'+'l'='eonl'
'eonl' inserted into 'night' is 'NEON LIGHT'.
'this' acts as a link.
Can you help me to learn more?
(Other definitions for neon light that I've seen before include "Advertising sign, perhaps" , "Type of fluorescent lamp" , "What may shine" , "Type of illumination" , "Source of illumination" .)