18 across
Maximum
break
adds
up
to
12
(3,4,5)
Maximum break adds up to 12 (3,4,5)
hints
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checks
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The whole clue is the definition (& lit or double definition).
How the clue works
The whole clue is a cryptic definition of Maximum break adds up to 12: 147 is the maximum possible break in snooker, spelled out as ONE FOUR SEVEN. The confirming twist is that its digits 1 + 4 + 7 add up to 12, so the '12' is used literally rather than as a cross-reference.