The most common and most likely answer to this crossword clue is the 5 plus 3 letter word RIVER BED
I've seen this crossword clue in Independent
R
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Buddy explains!
The straight part
We know that "Lower part of the Congo possibly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lower part of the congo possibly = river bed.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does RIVER BED mean?
RIVER BED noun- The path where a river runs, or where a river once ran; the bottom earthen part of a river, not including the riverbanks.