Possible Clues:

• Mole
• Mole, in spydom
• Mole, perhaps
• Inside man in spydom
• Bizarre legend about Philby, for one
• Spy
• Spy novel figure (2 words)
• One not to be trusted in spydom
• Mole—and, read slightly differently, what 17-, 24-, 38- or 51-Across is
• Treacherous spy