The most common and most likely answer to this crossword clue is the 3 plus 4 plus 5 letter word THE LAST LAUGH
T
H
E
L
A
S
T
L
A
U
G
H
More clues leading to the result THE LAST LAUGH
Buddy explains!

The straight part
We know that "Ultimate success" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ultimate success = the last laugh.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.