X marks this first in English, zero in the three Rs (5)
I believe the answer is:
error
'x marks this' is the definition.
(an error may be marked with a cross)
'first in english zero in the three rs' is the wordplay.
'three rs' means three replacements for 'r'.
'first in english' becomes 'e' (1st letter of 'english').
'zero' becomes 'O' (O resembles 0).
'in the' indicates putting letters inside.
'r'+'r'+'r'='rrr'
'o' placed inside 'rrr' is 'rror'.
'e'+'rror'='ERROR'
(Other definitions for error that I've seen before include "Bug" , "wrong" , "Boo-boo" , "Mistake from terrorist" , "Faux pas" .)