Colin was cleaning out in the attic

[GoesStation]

Exactly. I deal with this issue very often. I always err towards one word, but as GoesStation says, sometimes it looks a little awkward on the page. I think clearout looks fine, though.

Not being used in American English, it doesn't have enough currency to justify losing the hyphen. For clarity in American English, I'd write it as a clear-out. I'd want to provide more context so my (American) reader could guess what it means.