The most common and most likely answer to this crossword clue is the 12 letter word GREAT-NEPHEW
I've seen this crossword clue in The Guardian
G
R
E
A
T
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N
E
P
H
E
W
Buddy explains!
The straight part
We know that "Kinsman" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case kinsman = great-nephew.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
GREAT-NEPHEW noun- A grandson of one's sibling; a son of one's nephew or niece. (Brother's grandson: fraternal grandnephew. Sister's grandson: sororal grandnephew.)