The defense began its case Monday in the trial of a former Chelsea selectwoman charged with extortion, tax evasion and fraud. 55-year-old Carole Swan is accused of taking about $20,000 in kickbacks from a contractor while she served as a selectwoman. Prosecutors say she also inflated the cost of road projects awarded to her husband’s construction business and failed to report almost $700,000 in income.Swan’s lawyers say she was an abused wife who only followed the orders of her husband, Marshall Swan. He’ll be tried separately in the case. Carole Swan’s mother reportedly took the stand first Monday, recounting her daughter’s domestic abuse. A former Chelsea town clerk also testified Carole Swan said she was conducting an investigation into the contractor she allegedly bilked.Prosecutors called more than 100 witnesses before concluding earlier in the day.The trial is scheduled to wrap up by the end of the week.