There are rules for determining just what it is (6)
I believe the answer is:
length
'it is' is the definition.
The definition suggests a singular noun which matches the answer.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for length that I've seen before include "A bit of swimming" , "Duration or distance" , "Duration, extent" , "Extension in space" , "Extent - distance" .)