Visionary American attached to Yard in challenge (10)
I believe the answer is:
daydreamer
'visionary' is the definition.
(thesaurus)
'american attached to yard in challenge' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'attached to' means one lot of letters go next to another.
'yard' becomes 'yd' (abbreviation).
'in' indicates putting letters inside.
'challenge' becomes 'dreamer' (I can't explain this - if you can you should believe this answer much more).
'a'+'yd'='ayd'
'ayd' put within 'dreamer' is 'DAYDREAMER'.
Can you help me to learn more?
(Other definitions for daydreamer that I've seen before include "One letting their mind wander" , "Wishful thinker" , "One distracted from the present" , "One in a reverie" , "Fantasist" .)