Poetic measure (4)
I believe the answer is:
iamb
'poetic measure' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for iamb that I've seen before include "Short-long metric foot" , "Poet's foot" , "Line of verse" , "Short-long metrical foot" , "Part of a pentameter?" .)