15 across
System
of
rules
requiring
constant,
dedicated
work?
(4)
System of rules requiring constant, dedicated work? (4)
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The definition is at the beginning of the clue.
How the clue works
System of rules is your definition here. Start with C, the standard abbreviation for a constant, then tack on ODE, that dedicated piece of writing you'd call a "work" — stick them together, C then ODE, and there's your answer.