The most common and most likely answer to this crossword clue is the 6 letter word MIXING
I've seen this crossword clue in The Guardian
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More clues leading to the result MIXING
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 = mixing.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 MIXING mean?
MIXING verb- To stir together.
- To combine (items from two or more sources normally kept separate).
- To form by mingling; to produce by the stirring together of ingredients; to concoct from different parts.
- To blend by the use of a mixer (machine).
- To combine (several tracks).
- To produce a finished version of (a recording).
- To unite with in company; to join; to associate.
- The act, or the result of making a mixture.