Thanks orson.
For 6d, Pat + Stern gives Astern with T extra. But why is "to row" in the clue?
11d. I finally parsed it as STreet for thoroughfare. And Pile (Building) inside.
So got Spilet. If E is extra letter for 11a., it becomes Spilt.
Past tense of Spell would be Spelt. So I becomes extra?
BTW, my 5d is Dees. I did not bother to parse it. Deep would be incomprehensible. Why articles, I don't know.