Meaning of 'to have at it'
"Octavia had been trimming the dead heads off the roses. She’d said they needed pruning back for the winter, and I’d told her to have at it. My grandmother had been the rose person in our household, and she hadn’t let me lay a finger on them unless they needed spraying for aphids. That had been one of my jobs."
It is in Chapter 18 of From Dead to Worse. Do you know what does it mean by ' to have at'? Moreover, can you provide another example, using this expression? Thanks.