The most common and most likely answer to this crossword clue is the 9 plus 6 letter word BUTTERFLY EFFECT
B
U
T
T
E
R
F
L
Y
E
F
F
E
C
T
More clues leading to the result BUTTERFLY EFFECT
Buddy explains!
The straight part
We know that "R/ /small causes leading to big events" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case r/ /small causes leading to big events = butterfly effect.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.