The most common and most likely answer to this crossword clue is the 12 letter word CULMINATIONS
I've seen this crossword clue in Financial Times
C
U
L
M
I
N
A
T
I
O
N
S
Buddy explains!

The straight part
We know that "final stages" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case final stages = culminations.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 CULMINATIONS mean?
CULMINATIONS noun- The attainment of the highest point of altitude reached by a heavenly body; passage across the meridian; transit.
- Attainment or arrival at the highest pitch of glory, power, etc.