Related Clues:

• Mole, e.g.
• Engage in espionage
• Cloak-and-dagger sort
• Univ. requirement
• ___-fi (film type)
• Physics, for one: Abbr.
• 007, e.g.
• Keep tabs (on)
• CIA worker
• Faraday's forte: abbr.
• Observe furtively
• Chem. or bio.
• Mole or plant
• Plant, perhaps
• Seek intelligence, in a way