。。
Do some work already. It'll help your diet.
Literal
A-little-[topic-は] work-[imperative]. Diet [to-に] become-[masculine-ぞ].
A sarcastic scolding. 働け is the bare imperative form of 働く ('work') — a rough, direct command. 少しは uses the topic particle は after 少し to mean 'at least a little' (contrasting with 'not at all'). ダイエットになる 'becomes/serves as a diet' — the な-に-する・なる pattern where something 'becomes' or 'serves as' something else. ぞ is the masculine assertive sentence-ender.