Sentence:  But, to the extent that he remained insecure about his caregiving, particularly in settings outside the safe confines of home, Belinda had to <WSD>pick up the slack</WSD>. And the more she <WSD>picked up the slack</WSD>, the more he backed off and deferred to her. 
Meaning of the word marked with <WSD> tags:  To do work that would otherwise be left undone.