'Setter in mile race' (now proverbial Guardian feature?) (8)
I believe the answer is:
misprint
'proverbial guardian feature?' is the definition.
(the Guardian often contains typos)
'setter in mile race' is the wordplay.
'setter' becomes 'I' (the person who set this crossword).
'in' is an insertion indicator.
'mile' becomes 'm' (common abbreviation).
'race' becomes 'sprint' (I've seen this before).
'm'+'sprint'='msprint'
'i' placed within 'msprint' is 'MISPRINT'.
'now' is the link.
(Other definitions for misprint that I've seen before include "could make sinners of the winners!" , "Typo" , "Typical accident" , "bad spell perhaps" , "Literal interpretation" .)