Poem Tennyson had no heart to compose? (6)
I believe the answer is:
sonnet
'poem' is the definition.
(type of poem)
'tennyson had no heart to compose?' is the wordplay.
'had no heart' suggests removing the centre (I've seen 'having no heart' mean this).
'to compose?' indicates an anagram (I've seen 'composed' mean this).
'tennyson' with its middle taken out is 'tenson'.
'tenson' is an anagram of 'SONNET'.
(Other definitions for sonnet that I've seen before include "14-line poem" , "Shakespearean poem" , "A form of poetry" , "Fourteen-lined verse" , "The poem was sent on" .)