This state can't be treated lightly! (8)
I believe the answer is:
darkness
'this state can't be treated lightly' is the definition.
'darkness' can be an answer for 'state' (darkness is a kind of state). I am not certain of the rest of the definition.
'this state can't be' is the wordplay.
'this state' becomes 'dark' (dark is a kind of state).
'can't be' becomes 'ness' (I can't explain this - if you can you should believe this answer much more).
'dark'+'ness'='DARKNESS'
Can you help me to learn more?
(Other definitions for darkness that I've seen before include "'Hello . . . . . . . ., my old friend' (Simon and Garfunkel song)" , "Evil; lack of light" , "Absence of light" , "Black of night" , "Better to light a candle than to curse it, they say" .)