The most common and most likely answer to this crossword clue is the 8 letter word BUTTOCKS
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The straight part
We know that "rump" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rump = buttocks.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 BUTTOCKS mean?
BUTTOCKS noun- (usually in the plural) Each of the two large fleshy halves of the posterior part of the body between the base of the back, the perineum and the top of the legs.
- The convexity of a ship behind, under the stern.