Overwhelming disaster, it arising to obscure source of light in the morning (7)
I believe the answer is:
tsunami
'overwhelming disaster' is the definition.
'it arising to obscure source of light in the morning' is the wordplay.
'arising' shows that the letters should be reversed in order.
'to obscure' means one lot of letters goes inside another.
'source of light' becomes 'sun' (I've seen this before).
'in the morning' becomes 'am' (AM on a clock).
'it' written backwards gives 'ti'.
'sun'+'am'='sunam'
'ti' enclosing 'sunam' is 'TSUNAMI'.
(Other definitions for tsunami that I've seen before include "Huge tidal wave that causes great damage and loss of life" , "I am nuts! (anag)" , "Huge wave caused by earth tremor" , ""Harbour wave"" , "Large tidal wave" .)