The most common and most likely answer to this crossword clue is the 4 plus 2 plus 4 letter word PACK OF LIES
I've seen this crossword clue in The Guardian
P
A
C
K
O
F
L
I
E
S
Buddy explains!
The straight part
We know that "This isn’t to be trusted" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case this isn’t to be trusted = pack of lies.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.