Another cryptic crossword clue, maybe harder than the last one: though I normally get it wrong trying to predict how easy/hard clues are.
Biggest deep water hole gives light solution 
I realize this probably doesn't really fit the format hinted at in the last problem. Nevertheless, I'm thinking:
The Mariana Trench (in particular the Challenger Deep) is the deepest point on Earth's seabed (though as the Wikipedia article points out, in terms of depth below sea level at that point, not in terms of closest point to the Earth's center). As such, it would be very dark because of water absorbing light - so if light were a "problem" for you, the Mariana trench could be a "solution" (if not a very practical one for humans). And the name does have 7 letters.
so I want to put something in a discussion to explain the answer a little bit more than it is already described in the answers - but I have to do this in an answer to prevent spoilers...
So taking 'Biggest deep water hole gives light solution ' we replace Biggest by Max and 'deep water hole' by well and we get 'Maxwell gives light solution', which is noun verb noun noun (or could be noun verb adjective noun), which I think works -- because Maxwell's equations give the solution to the electromagnetic wave that is light ans so Maxwell gives light solution. ---- note the clue is also written like this because solution could also be a liquid and earlier in the clue we have 'deep water hole' so although it is a 'red herring' it makes sort of sense that a solution (or liquid) might come from a deep water hole. I think this is a bit of a tradition with cryptic clues to make them have a meaning completely unrelated to the actual answer. BUT PLEASE COMMENT if you think I have got this wrong or have another opinion.