Sentence:  -- "I suppose you wouldn't like to do a locum for a month on the South coast? Three guineas a week with board and lodging." -- "I wouldn't mind," said Philip. -- "It's at Farnley, in Dorsetshire. Doctor South. You'd have to go down at once; his assistant has developed mumps. I believe it's a very pleasant place." There was something in the secretary's manner that puzzled Philip. It was a little doubtful. -- "What's the <WSD>crab</WSD> in it?" he asked. 
Meaning of the word marked with <WSD> tags:  A defect in an outwardly normal object that may render it inconvenient and troublesome to use.