Leading motorsport article upset Green (4)
I believe the answer is:
naif
'green' is the definition.
(similar in meaning)
'leading motorsport article upset' is the wordplay.
'leading' means one lot of letters go next to another.
'motorsport' becomes 'if' (I can't justify this - if you can you should believe this answer much more).
'article' becomes 'an' (English indefinite article).
'upset' says the letters should be written backwards.
'an' reversed gives 'na'.
'if' after 'na' is 'NAIF'.
Can you help me to learn more?
(Other definitions for naif that I've seen before include "wide-eyed" , "Green" , "Innocent" , "Inexperienced" , "Unworldly" .)