The most common and most likely answer to this crossword clue is the 5 plus 10 letter word HEART TRANSPLANT
I've seen this crossword clue in The Guardian
H
E
A
R
T
T
R
A
N
S
P
L
A
N
T
Buddy explains!
The straight part
We know that "life-changing procedure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case life-changing procedure = heart transplant.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.