Japan maybe supporting auto plant (9)
I believe the answer is:
carnation
'plant' is the definition.
(carnation is a flower)
'japan maybe supporting auto' is the wordplay.
'japan maybe' becomes 'nation' (I am not sure about this - if you are sure you should believe this answer much more).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'auto' becomes 'car' (synonyms).
'nation' after 'car' is 'CARNATION'.
Can you help me to learn more?
(Other definitions for carnation that I've seen before include "Flower like pink" , "Feature of many a wedding" , "Anoint car with the strange flower" , "Pink - garden flower" , "Oscar Wilde wore a green one" .)