A certain something penned by excellent poet (7)
I believe the answer is:
sitwell
'poet' is the definition.
(I've seen this before)
'a certain something penned by excellent' is the wordplay.
I cannot really see how this works, but
'something' could be 'it' (I've seen this in other clues) and 'it' is found within the answer.
'excellent' could be 'swell' (I've seen this before) and 'swell' is present in the leftover letters.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sitwell that I've seen before include "Writers Edith, Osbert" , "Dame Edith --, English poet" , "Female poet" , "English poet and critic, d. 1964" , "Literary Dame" .)