A serious error in English is foolish (7)
I believe the answer is:
asinine
'foolish' is the definition.
(similar in meaning)
'a serious error in english' is the wordplay.
'serious error' becomes 'sin' (I've seen this before).
'english' becomes 'e' (abbreviation).
'a'+'sin'+'in'+'e'='ASININE'
'is' acts as a link.
(Other definitions for asinine that I've seen before include "Absurd" , "Foolish like donkey" , "In the nature of supposedly stupid animal" , "simple" , "Stubborn" .)