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' means to remove the middle letters.
'new' indicates anagramming the letters.
'tennyson' with its middle taken out is 'tenson'.
'tenson' with letters rearranged 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" .)