Bore is quietly intense, not loud (6)
I believe the answer is:
pierce
'bore' is the definition.
(thesaurus)
'quietly intense not loud' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'intense' becomes 'fierce' (similar in meaning).
'not' is a deletion indicator.
'loud' becomes 'f' (short for forte - 'loud' in music).
'fierce' with 'f' taken away is 'ierce'.
'p'+'ierce'='PIERCE'
'is' acts as a link.
(Other definitions for pierce that I've seen before include "Perforate" , "Penetrate with a sharp instrument" , "Make hole in with sharp object" , "cut to the quick" , "Cavalry sword" .)