Related Clues:

• Engine noise
• Be busy, as a bee
• Wavy lines, in the comics
• Polecat's defense
• Essence
• Olfactory input
• Olfactory offense
• Lilac, e.g.
• See 25-Across
• Stink
• Harmful gas
• Carbon monoxide's lack
• Horrid smell
• Lead for a Lab
• Whiff