Part of theatre in which 200 see captivating unknown play (with no director) (9)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

( CC [Roman numerals for 200] + LO [behold; see]) containing [captivating] Y (letter frequently used to denote an unknown value in an equation) + d RAMA (play) excluding (with no) D (director)