The most common and most likely answer to this crossword clue is the 10 letter word COMPOUNDER
I've seen this crossword clue in The Guardian
C
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More clues leading to the result COMPOUNDER
Buddy explains!
The straight part
We know that "mixer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mixer = compounder.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 COMPOUNDER mean?
COMPOUNDER noun- A person who compounds (mixes ingredients, and tests the result)
- One who attempts to bring persons or parties to terms of agreement, or to accomplish ends by compromises.
- One who compounds a debt, obligation, or crime.
- One at a university who pays extraordinary fees for the degree he is to take.
- A Jacobite who favoured the restoration of James II, on condition of a general amnesty and of guarantees for the security of the civil and ecclesiastical constitution of the realm.