Loud American taken in by genuine rejection (7)
I believe the answer is:
refusal
'rejection' is the definition.
(I've seen this before)
'loud american taken in by genuine' is the wordplay.
'loud' becomes 'f' (A musical term).
'american' becomes 'us'.
'taken in by' indicates putting letters inside.
'genuine' becomes 'real' ('real' is associated in meaning with 'genuine').
'f'+'us'='fus'
'fus' placed within 'real' is 'REFUSAL'.
(Other definitions for refusal that I've seen before include "Rejection of an offer" , "Act of a horse stopping short at a jump" , "Negative response" , "Rebuff" , "(Horse) failure to jump" .)