Ultimately benighted, the writer is stupid (3)
I believe the answer is:
dim
'stupid' is the definition.
(similar in meaning)
'ultimately benighted the writer is' is the wordplay.
'ultimately' says to take the final letters.
'the writer is' becomes 'I'm' ('I am', referring to the person who wrote the clue).
The final letter of 'benighted' is 'd'.
'd'+'im'='DIM'
(Other definitions for dim that I've seen before include "Obscure, not bright" , "Sombre" , "Not bright, cloudy" , "& 26 Chinese food selection" , "reduce volume" .)