The most common and most likely answer to this crossword clue is the 13 letter word REVOLUTIONISE
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result REVOLUTIONISE
Buddy explains!
The straight part
We know that "to make great changes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to make great changes = revolutionise.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.
What does REVOLUTIONISE mean?
REVOLUTIONISE verb- To change radically or significantly, as in a revolution.