The most common and most likely answer to this crossword clue is the 13 letter word CONSOLIDATION
C
O
N
S
O
L
I
D
A
T
I
O
N
More clues leading to the result CONSOLIDATION
Buddy explains!

The straight part
We know that "merger" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case merger = consolidation.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 CONSOLIDATION mean?
CONSOLIDATION noun- The act or process of consolidating, making firm, or uniting; the state of being consolidated
- The combination of several actions into one.
- A solidification into a firm dense mass. It is usually applied to induration (swelling or hardening of normally soft tissue) of a normally aerated lung.