The most common and most likely answer to this crossword clue is the 6 letter word CRASIS
C
R
A
S
I
S
Buddy explains!
The straight part
We know that "Mingling of vowels" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mingling of vowels = crasis.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 CRASIS mean?
CRASIS noun- One's constitution; the balance of humours in a person's body.
- A mixture or combination.
- External vowel sandhi; contraction of a vowel or diphthong at the end of a word with a vowel or diphthong at the start of the following word.