The most common and most likely answer to this crossword clue is the 6 letter word DOUBTS
I've seen this crossword clue in The Guardian
D
O
U
B
T
S
Buddy explains!

The straight part
We know that "Mistrusts" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mistrusts = doubts.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 DOUBTS mean?
DOUBTS noun- Disbelief or uncertainty (about something); a particular instance of such disbelief or uncertainty.
- A point of uncertainty; a query.
- To be undecided about; to lack confidence in; to disbelieve, to question.
- To harbour suspicion about; suspect.
- To anticipate with dread or fear; to apprehend.
- To fill with fear; to affright.
- To dread, to fear.