Have at it

三年前に東野圭吾の "The Devotion of Suspect X" を読み大変良かったと記憶していますが、当時は同作家の英訳本は他にはなかった（江戸川区の図書館には）のですが、東野圭吾の英訳本は2冊増えていて、その一つの "JOURNEY UNDER THE MIDNIGHT SUN" を読むことにしました。

'Hey, Enjoying your day off?' the older detective, a man by the name of Kobayashi, said. He was two years Sasagaki's senior. The younger man, Detective Koga, had joined Homicide less than a year before
'I had a had feeling when I woke up this morning,' Sasagaki said.
'Wish I'd been wrong for a change.' He lowered his voice. 'How's the old man's mood?'
Kobayashi frowned and shook his head. Koga gave a wry smile.
'That's what I figured,' Sasagaki said. 'Well, no rest for the wicked. What's he up to in there?'
'Dr Matsuno just got here.'
'Right.'
Kobayashi cleared his throat. 'We're going to take a look around outside, OK?'
'Have at it.'
Sasagaki watched the two leave.

口語的な表現の "Have at it" は文脈からして「そうしたら」の意味だと思いますが、辞書で確認します。
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs: Start doing it.; Start eating your food. John: Here's your hamburger. Have at it.
・Urban Dictionary: Start doing something. Get down to it.：We are ready to start, let's have at it.