Okay, it was word order that confused me. You're right Alex, I was stupid to overlook grammatical cases.

So:-

'It flew off, but the anguish remained unexplained, because another brief thought could not really explain it.'

Which, retaining the order of clauses in the original, becomes:-

'It flew off, but the anguish remained unexplained, since it could not well BE EXPLAINED BY ...another brief thought:-'

I need to remember to look at case first, then word order second, THEN work out how to express this in English word order. Not the other way around, or whatever I did with this sentence...

Thanks both.