Writer of rhymes, say, is Tennyson (8)
I believe the answer is:
essayist
'writer' is the definition.
(essayist is a kind of writer)
'of rhymes say is tennyson' is the wordplay.
'of' indicates the answer is hidden within the clue.
'ESSAYIST' is hidden in the letters of 'rhymes say is tennyson'.
(Other definitions for essayist that I've seen before include "I write" , "Writer such as Addison" , "Writer of shorter pieces" , "Lamb, perhaps" , "Writer of little prose pieces" .)