The most common and most likely answer to this crossword clue is the 5 letter word SLAVE
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The straight part
We know that "Work very hard" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case work very hard = slave.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SLAVE mean?
SLAVE noun- A person who is held in servitude as the property of another person, and whose labor (and often also whose body and life) is subject to the owner's volition and control.
- A drudge; one who labors or is obliged (e.g. by prior contract) to labor like a slave with limited rights, e.g. an indentured servant.
- An abject person.
- One who has no power of resistance (to something), one who surrenders to or is under the domination (of something).
- (BDSM) A submissive partner in a BDSM relationship who (consensually) submits to (sexually and/or personally) serving one or more masters or mistresses.
- A sex slave, a person who is forced against their will to perform, for another person or group, sexual acts on a regular or continuing basis.
- A device (such as a secondary flash or hard drive) that is subject to the control of another (a master).
- To work as a slaver, to enslave people.
- To work hard.
- To place a device under the control of another.