Extremely dark at first, gas light turned up (2,3)
I believe the answer is:
no end
'extremely' is the definition.
(I've seen this in another clue)
'dark at first gas light turned up' is the wordplay.
'at first' says to take the initial letters.
'gas light' becomes 'neon' (I've seen this in another clue).
'turned up' says the letters should be written backwards.
The first letter of 'dark' is 'd'.
'd'+'neon'='dneon'
'dneon' backwards is 'NO END'.
(Other definitions for no end that I've seen before include "detailed?" , "Very much (colloq.)" , "Very much (unfinished?)" , "Non-stop" , "Without limit" .)