The most common and most likely answer to this crossword clue is the 12 letter word SLEDGEHAMMER
I've seen this crossword clue in The Guardian
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More clues leading to the result SLEDGEHAMMER
- Insult film company's OTT 'Nutcracker'?
- Tool has blade: hard to smuggle into jail
- Heavy tool required to break emerald and hard gems
- Creep hard to put in jail is a big hitter
- Ridiculous means by which Brazil may be entered?
- Bound to get hard in prison, this employed with force!
- Advantage having husband in prison? It strikes a heavy blow
- Tool for hard labour damaged hedge in prison
- Big tool shelf fixed by phoney fellow
- In prison, creep’s hard to beat
Buddy explains!

The straight part
We know that "Tool for hard labour" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tool for hard labour = sledgehammer.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 SLEDGEHAMMER mean?
SLEDGEHAMMER noun- A hammer that consists of a large, heavy, broad and flat block of metal (the head) attached to a handle typically 0.5 meter to 1 meter long. The sledgehammer's design is meant to allow it to be swung powerfully, and to distribute force over a wide area upon impact.
- To strike with a sledgehammer.