Poet's foot (4)
I believe the answer is:
'poet's foot' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for iamb that I've seen before include "Metrical foot of one short, one long syllables" , "Poet's foot" , "Part of a pentameter?" , "Short-long metrical foot" , "Line of verse" .)