be boxing clever

To be acting skillfully and cunningly or deceitfully to achieve a desired outcome. Primarily heard in UK. I wasn't boxing clever at the time, so it was easy for the headmaster to see that I had broken the rules.

If someone is boxing clever, they are being very clever and careful in the way they behave in a difficult situation, so that they can get an advantage for themselves. By boxing clever with your personal tax allowances you could save £900 a year.