Fool previously grasping an element of poetry (9)
I believe the answer is:
assonance
'element of poetry' is the definition.
(assonance is a poetic technique)
'fool previously grasping an' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'previously' becomes 'once' (I've seen this before).
'grasping' indicates putting letters inside.
'once' going around 'an' is 'onance'.
'ass'+'onance'='ASSONANCE'
(Other definitions for assonance that I've seen before include "The repetition of similar-sounding words or syllables" , "Resemblance of sound between syllables of nearby words" , "Audible agreement" , "Partial correspondence" , "Correspondence in sound" .)