In this context, the tax credit would be an amount of money which the tax authority paid to the company. We can guess that Elmo paid tax amounts regularly throughout the year which had been calculated on the basis of a certain sum of predicted profits. At the end of the year, Elmo had made a big loss and so had overpaid £12,000 of income tax. The tax authority refunded or credited this sum to Elmo. (NB In the UK, income tax is paid by individuals and corporation tax is paid by companies.)