The most common and most likely answer to this crossword clue is the 10 letter word MORGANATIC
I've seen this crossword clue in The Guardian
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More clues leading to the result MORGANATIC
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The cryptic/wordplay part
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What does MORGANATIC mean?
MORGANATIC adjective- Designating a marriage (or the wife involved) between a man of higher rank and a woman of lower rank, often having various legal repercussions (typically that such a wife has no claim on the husband's possessions or title). It was not an aspect of English law, but was common in other royal houses, especially in Germany.