Poetic rhythm (5)
I believe the answer is:
metre
'poetic rhythm' is the definition.
(I know that metre is a type of poetic rhythm)
This is all the clue.
(Other definitions for metre that I've seen before include "Basic unit of length" , "39.37 inches" , "Approximately 39 inches" , "Poetic rhythm" , "three and a bit of a fourth" .)