Crossword Genius

There are rules for determining just what it is (6)

Ross

I believe the answer is:

length

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'it is' is the definition.
The definition suggests a singular noun which matches the answer.

I don't understand the rest of the clue.

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(Other definitions for length that I've seen before include "A bit of swimming" , "Duration or distance" , "Duration, extent" , "Extension in space" , "Extent - distance" .)

I've seen this clue in the Evening Standard.
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