Loud organ causes apprehension (4)
I believe the answer is:
fear
'apprehension' is the definition.
(I've seen this before)
'loud organ' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'organ' becomes 'ear' (the ear is an organ).
'f'+'ear'='FEAR'
'causes' acts as a link.
(Other definitions for fear that I've seen before include "Anxiety caused by danger" , "awe" , "Consternation" , "Dread, great anxiety" , "Alarm, reverence" .)