A writer from Wales, say, is Thomas (8)
I believe the answer is:
essayist
'a writer' is the definition.
(I know that essayist is a type of writer)
'from wales say is thomas' is the wordplay.
'from' indicates the answer is hidden within the clue.
'ESSAYIST' is hidden in the letters of 'wales say is thomas'.
(Other definitions for essayist that I've seen before include "Writer such as Addison" , "Writer of shorter pieces" , "Lamb, perhaps" , "Bacon maybe" , "Montaigne, say" .)