The most common and most likely answer to this crossword clue is the 9 letter word INTERBRED
I
N
T
E
R
B
R
E
D
More clues leading to the result INTERBRED
Buddy explains!

The straight part
We know that "Like a 6 dn" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like a 6 dn = interbred.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 INTERBRED mean?
INTERBRED verb- To breed or reproduce within an isolated community.
- To breed or reproduce within a heterogenous community, the products of which produce hybrids.