The most common and most likely answer to this crossword clue is the 13 letter word MISMAR[R]IAGE
M
I
S
M
A
R
[
R
]
I
A
G
E
Buddy explains!

The straight part
We know that "not a good match" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not a good match = mismar[r]iage.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.
Crossword Answer definitions
MISMAR[R]IAGE noun- Bad or unsuitable marriage.