Cryptic crossword clue
Siege ruins ground that is unique 3,7

The most common and most likely answer to this crossword clue is the 3 plus 7 letter word SUI GENERIS

I've seen this crossword clue in The Guardian
S
U
I
G
E
N
E
R
I
S

Buddy explains!

The straight part
We know that "unique" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unique = sui generis.
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.

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