Sentence:  on one side of her lay a pair of carpet slippers and on the other a ball of red wool, the leading filament of which she would tug at every now and then with the immemorial elbow jerk of a Zemblan knitter to give a turn to her yarn <WSD>clew</WSD> and slacken the thread. 
Meaning of the word marked with <WSD> tags:  (yarn)  A ball of thread or yarn.