The most common and most likely answer to this crossword clue is the 6 letter word STOLON
S
T
O
L
O
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Buddy explains!
The straight part
We know that "Runner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case runner = stolon.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 STOLON mean?
STOLON noun- A shoot that grows along the ground and produces roots at its nodes; a runner.
- A structure formed by some colonial organisms from which offspring are produced by budding, found in bryozoans, pterobranchs, some corals, and other invertebrates.
- A hypha that acts as a runner, connecting sporangiophores.