Clark previously confirmed that he would helm two instalments for the next series.
He wrote on his Twitter feed: "I'm scared filming this [episode]. The kids are gonna freak."
Clark later added that he was preparing to wrap production on his two-episode stint.
He continued: "Big finale today. Complex. Lots to co-ordinate. But it's looking seriously gorgeous."
He also praised the performance of star Matt Smith, but refused to confirm the identities of any guest actors.
"Matt's performance [is] a joy," he said. "Wonderfully talented actor. Sadly [I] can't reveal anything about guests, except that I'm delighted."
It was previously reported that actors Daniel Mays, Emma Cunniffe and Jamie Oram have been cast in the new series.