The most common and most likely answer to this crossword clue is the 6 letter word SLIPPY
S
L
I
P
P
Y
Buddy explains!

The straight part
We know that "untrustworthy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case untrustworthy = slippy.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 SLIPPY mean?
SLIPPY adjective- (slightly informal) Slippery.
- Spry, nimble.