The most common and most likely answer to this crossword clue is the 1 plus 4 plus 6 plus 2 plus 4 letter word A FINE KETTLE OF FISH
I've seen this crossword clue in The Times
A
F
I
N
E
K
E
T
T
L
E
O
F
F
I
S
H
Buddy explains!
The straight part
We know that "troublesome thing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case troublesome thing = a fine kettle of fish.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.