The most common and most likely answer to this crossword clue is the 6 letter word DIBBLE
I've seen this crossword clue in The Guardian
D
I
B
B
L
E
More clues leading to the result DIBBLE
Buddy explains!

The straight part
We know that "Gardener’s tool" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gardener’s tool = dibble.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 DIBBLE mean?
DIBBLE noun- A pointed implement used to make holes in the ground in which to set out plants or to plant seeds.
- To make holes or plant seeds using, or as if using, a dibble.
- To use a dibble; to make holes in the soil.
- To dib or dip frequently, as in angling.
- (originally Manchester) A police officer.
- (originally Manchester) Preceded by the: the police.