The most common and most likely answer to this crossword clue is the 7 letter word KNAVISH
I've seen this crossword clue in Financial Times
K
N
A
V
I
S
H
Buddy explains!
The straight part
We know that "Dishonest/a bit like Jack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dishonest/a bit like jack = knavish.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 KNAVISH mean?
KNAVISH adjective- Having the characteristics of a knave; mischievous, roguish, waggish, rascally or impertinent