Item of poetry penned by William Blake (4)
I believe the answer is:
iamb
'item of poetry' is the definition.
(iamb is a stressed syllable plus an unstressed one)
'penned by william blake' is the wordplay.
'penned by' says the answer is hidden in the clue.
'IAMB' can be found hidden inside 'william blake'.
(Other definitions for iamb that I've seen before include "Short-long metric foot" , "Line of verse" , "Metrical unit" , "Metrical foot of one short, one long syllables" , "Poet's foot" .)