One prepares chap at a wedding in America (9)
I believe the answer is:
groomsman
'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'prepares chap at a wedding in america' is the wordplay.
I cannot really see how this works, but
'prepares' could be 'grooms' ('groom' can be a synonym of 'prepare') and 'grooms' is found within the answer.
'chap' could be 'man' (chap can mean a man) and 'man' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for groomsman that I've seen is " Male wedding attendant".)