Related Clues:

• Fingerprint or dropped handkerchief, say
• Toxic stuff
• This, for example
• Kind of pen
• Detective's need
• Print, maybe
• It may be found with a magnifying glass
• Guiding light?
• Inkling
• Fingerprint, perhaps
• Sleuth's find
• Airhead's lack?
• Footprint, maybe
• Skull-and-crossbones stuff
• This is one