The most common and most likely answer to this crossword clue is the 7 letter word DOLMENS
I've seen this crossword clue in The Guardian
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More clues leading to the result DOLMENS
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The straight part
We know that "tombs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tombs = dolmens.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.
Crossword Answer definitions
DOLMENS noun- A prehistoric megalithic tomb consisting of a capstone supported by two or more upright stones, most having originally been covered with earth or smaller stones to form a barrow.
- More generally, any megalithic tomb, including passage graves and wedge tombs.