Fearful of a loud attack (6)
I believe the answer is:
afraid
'fearful' is the definition.
(similar in meaning)
'a loud attack' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'attack' becomes 'raid' (raid is a kind of attack).
'a'+'f'+'raid'='AFRAID'
'of' acts as a link.
(Other definitions for afraid that I've seen before include "Filled with concern" , "Feeling anxiety or fear" , "Fearful, anxious" , "feeling scared" , "Filled with fear" .)