Sentence:  We drove back to the office with some <WSD>concern</WSD> on my part at the prospect of so large a case. Sunning himself on the board steps, I saw for the first time Mr. Farquhar Fenelon Cooke. He was dressed out in broad gaiters and bright tweeds, like an English tourist, and his face might have belonged to Dagon, idol of the Philistines. 
Meaning of the word marked with <WSD> tags:  That which affects one's welfare or happiness. A matter of interest to someone. The adposition before the matter of interest is usually over, about or for.