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' says to hollow out the word (remove centre letters).
'new' indicates an anagram.
'tennyson' with its middle removed is 'tenson'.
'tenson' is an anagram of '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" .)