The main operating subsidiary of National Australia Bank traded in the red in the half year to March 2011, financial statements for Clydesdale Bank published this week show.The loss for the half-year is due to NAB's decision to recognise the provision for mis-selling of consumer credit insurance in the March half-year. NAB had not resolved its accounting treatment of this item when it announced its group profit in early May.Clydesdale incurred a loss of £43 million in the half, following a net profit of £32 million in the September 2010 half, and £4 million in the March 2010 half.Last month, NAB disclosed that its earnings for its UK banking unit, including wholesale banking and wealth management, were £77 million for the half year.On a pro forma basis that figure ignores the insurance provision. Clydesdale reported a net profit of £45 million in the latest half, marginally down on the six months prior.NAB also chipped in £200 million in new capital to Clydesdale, in the form of preference shares, over the latest half, though it reduced subordinated debt by £250 million.Over the last 18 months, NAB has had to bolster the Clydesdale balance sheet with £670 million in additional capital for a business with no growth in assets.