I had some trouble with 21 Across yesterday:
Sharp turns in quiet Scottish river valley (6)
Well, “sh” for quiet is easy. And inside that “tart” turning backwards is OK. I had never heard of strath. But it makes sense. Strathclyde. Strathalbyn. It had just never dawned on me to wonder what a strath is. I look it up and (duh) it is a Scottish word meaning a river valley.
The locals here say Strath as short for the local town of Strathalbyn. I will think of it in a new light now.