Heartless Tennyson's new poem (6)
I believe the answer is:
sonnet
'poem' is the definition.
(type of poem)
'heartless tennyson's new' is the wordplay.
'heartless' suggests removing the centre.
'new' is an anagram indicator.
'tennyson' with its middle removed is 'tenson'.
'tenson' anagrammed gives 'SONNET'.
(Other definitions for sonnet that I've seen before include "Shelley's Ozymandias, for example" , "A form of poetry" , "Form of poem" , "Fourteen-line 11" , "Poetic fourteen-liner" .)