The most common and most likely answer to this crossword clue is the 7 letter word AMALGAM
I've seen this crossword clue in The Daily Telegraph
A
M
A
L
G
A
M
More clues leading to the result AMALGAM
- Line in magazine included in a morning mix
- Mixture of aluminium in molten rock on the turn
- Asian priest returned to school compound
- Priest's backing mostly willing union
- Material United Artists may argue lacks global appeal, missing their premieres
- Filling the morning arranging gala on First of May
- Magma confused with 50% lava mixture
- Disheartened, several limbs broken by a combination of things
- Mixture of hot rock rolled to cover aluminium
- A bad French publication set up merger
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 = amalgam.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 AMALGAM mean?
AMALGAM noun- An alloy containing mercury.
- A combination of different things.
- One of the ingredients in an alloy.
- To amalgamate.