Loud American with son causing bother (4)
I believe the answer is:
fuss
'bother' is the definition.
(I know that bother can be written as fuss)
'loud american with son' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'american' becomes 'us' (abbreviation for United States).
'with' says to put letters next to each other.
'son' becomes 's' (genealogical abbreviation for son).
'f'+'us'+'s'='FUSS'
'causing' acts as a link.
(Other definitions for fuss that I've seen before include "Worry without reason" , "Agitate" , "Hoo-ha" , "Bother, ado" , "Unnecessary activity or bother" .)