Standard of length and rhythm that's observed throughout except in this particular clue (5)
I believe the answer is:
metre
'standard of length' is the definition.
'metre' can be an answer for 'length' (I've seen this before). I'm not sure about the 'standard of' bit.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for metre that I've seen before include "Poetic rhythm" , "Beat pattern (mus.)" , "three and a bit of a fourth" , "39.37 inches" , "100cm" .)