Makes an idiotic error, initially, supplying homonym (8)
I believe the answer is:
namesake
'homonym' is the definition.
'makes an idiotic error initially' is the wordplay.
'idiotic' is an anagram indicator.
'initially' indicates taking the first letters.
The first letter of 'error' is 'e'.
'makes'+'an'='makesan'
'makesan' anagrammed gives 'namesak'.
'namesak'+'e'='NAMESAKE'
'supplying' acts as a link.
(Other definitions for namesake that I've seen before include "One is called after someone" , "One addressed in the same way" , "Seek a man (anag) -- one with an intimate connection?" , "Appellation sharer" , "One with the same appellation" .)