Related Clues:

• Sound colour: black
• Tree said to be sound
• Absolutely straight
• Six feet (under the sea!)
• Measure of depth
• Exactly - in a vertical line
• Six feet
• Exactly - perpendicular
• Fathom - out-and-out
• Discover the meaning of - six feet
• Measure the depth
• Absolutely straight — sound
• Six feet from a swear word on wavelength
• Comprehend — a depth of six feet
• Make sense of breaking promise in broadcast wave