Clue: Agent offering Hobson's choice of guide on vacation, with a 9 (6,6)
Answer: GEORGE SMILEY
Explanation: Definition: Agent
Quite a bit going on here: 'Hobson's choice' is a choice of one thing, take it or leave it; GE ('**G**uid**E** on vacation') OR GE. 9A is GRIN, giving SMILEY ('with a 9'). The 'agent' in the Circus is GEORGE SMILEY, the central character in several novels by John le Carré, the best known being *Tinker Tailor Soldier Spy.