Related Clues:

• Toils
• Is employed
• Breaks one's back, say
• Is in operating condition
• Opera
• Grinds
• Works like a dog
• Doesn't do the job easily
• Works a double shift, e.g.
• Works very hard
• Is operational
• Plies one's trade
• Everything, with "the"
• Is effective
• Works in the mine, say