American Tour de France winner mostly in yellow (5)
I believe the answer is:
lemon
'yellow' is the definition.
(lemon is a kind of yellow)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for lemon that I've seen before include "something defective" , "Fruit squeezed on Pancake Day?" , "leaving bitter taste?" , "Something worthless" , "something unattractive" .)