The most common and most likely answer to this crossword clue is the 2 plus 2 plus 2 plus 4 letter word UP TO NO GOOD
U
P
T
O
N
O
G
O
O
D
Buddy explains!
The straight part
We know that "on the fiddle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case on the fiddle = up to no good.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.