The most common and most likely answer to this crossword clue is the 6 letter word DIBBER
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More clues leading to the result DIBBER
Buddy explains!

The straight part
We know that "garden tool" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case garden tool = dibber.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 DIBBER mean?
DIBBER noun- A tool with a handle on one end and a point on the other, used in the garden to poke holes in preparation for planting seeds, bulbs, etc. Also known as a dibble or dib.
- One who dibs.