The most common and most likely answer to this crossword clue is the 10 plus 6 letter word HERBACEOUS BORDER
I've seen this crossword clue in The Guardian
H
E
R
B
A
C
E
O
U
S
B
O
R
D
E
R
Buddy explains!
The straight part
We know that "part of garden" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of garden = herbaceous border.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.