The writer Thomas Hardy languishes, greatly diminished (5)
I believe the answer is:
dylan
'the writer thomas' is the definition.
(poet and novelist Dylan Thomas)
'hardy languishes greatly diminished' is the wordplay.
'greatly diminished' indicates the answer is hidden within the clue (diminish the letters to reveal the hidden word).
'DYLAN' is hidden in the letters of 'hardy languishes'.
(Other definitions for dylan that I've seen before include "Bob --, US folk singer" , "Bob (singer) - Thomas (author)" , "... Thomas, Welsh writer" , "Great Welsh poet . . . . . Thomas" , "Bob (singer-songwriter) - Thomas (poet)" .)