Piece of Whitman — it's essentially about poem (7)
I believe the answer is:
sestina
'poem' is the definition.
(I've seen this before)
'piece of whitman it's essentially about' is the wordplay.
'piece of' says the answer is hidden in the clue.
'about' says the letters should be written backwards.
'anitses' is hidden within 'whitman its essentially'.
'anitses' in reverse letter order is 'SESTINA'.
(Other definitions for sestina that I've seen before include "Old verse form" , "six-line stanza" , "poetic structure" , "Form of poem" , "Bit of poetry" .)