Rhythmic foot (4)
I believe the answer is:
iamb
'rhythmic foot' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for iamb that I've seen before include "Short-long metrical foot" , "Metrical foot of one short, one long syllables" , "Poet's foot" , "Metrical unit" , "Short-long metric foot" .)