Loud American's nervous about a dream (7)
I believe the answer is:
fantasy
'a dream' is the definition.
(I've seen this before)
'loud american's nervous about' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'nervous' becomes 'antsy' (I've seen this in another clue).
'about' indicates putting letters inside.
'a' inserted inside 'antsy' is 'antasy'.
'f'+'antasy'='FANTASY'
(Other definitions for fantasy that I've seen before include "Fay tans badly and forms a mental image" , "Whimsical, imaginative, notion" , "Imagination unrestricted by reality" , "Daydream, illusion or fancy" , "Unreal idea" .)