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

The straight part
We know that "Line of planks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case line of planks = strake.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 STRAKE mean?
STRAKE noun- An iron fitting of a medieval cart wheel.
- A type of aerodynamic surface mounted on an aircraft fuselage to fine-tune the airflow.
- A continuous line of plates or planks running from bow to stern that contributes to a vessel's skin. (FM 55-501).
- A shaped piece of wood used to level a bed or contour the shape of a mould, as for a bell
- A trough for washing broken ore, gravel, or sand; a launder.
- A streak.
- To stretch.
- (sometimes with out or through) To delete or cross out; to scratch or eliminate.
- (physical) To have a sharp or sudden effect.
- To thrust in; to cause to enter or penetrate.
- (personal, social) To have a sharp or severe effect.
- To touch; to act by appulse.
- To take down, especially in the following contexts.
- To set off on a walk or trip.
- To pass with a quick or strong effect; to dart; to penetrate.
- To break forth; to commence suddenly; with into.
- To become attached to something; said of the spat of oysters.
- To make and ratify.
- To level (a measure of grain, salt, etc.) with a straight instrument, scraping off what is above the level of the top.
- To cut off (a mortar joint, etc.) even with the face of the wall, or inward at a slight angle.
- To hit upon, or light upon, suddenly.
- (sugar-making) To lade thickened sugar cane juice from a teache into a cooler.
- To stroke or pass lightly; to wave.
- To advance; to cause to go forward; used only in the past participle.
- To balance (a ledger or account).