The most common and most likely answer to this crossword clue is the 8 plus 7 letter word SWEEPING CHANGES
I've seen this crossword clue in The Daily Telegraph
S
W
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P
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G
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H
A
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S
More clues leading to the result SWEEPING CHANGES
Buddy explains!
The straight part
We know that "They had significant effect, benefiting Kingsley’s Tom" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they had significant effect, benefiting kingsley’s tom = sweeping changes.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.