This is a charade - multiple word parts are combined in sequence to build the answer.
Take the first letters of 'box office', then reverse 'managed', then add what a 'bill' becomes when 'passed'. The parts combine in order.
Answer: BONAR LAW | B[ox] + O[ffice] + reverse of RAN (managed) + LAW (a bill that has been passed). Andrew Bonar Law was briefly Prime Minister in 1922-1923.