Formidable loud organ — remarkable! (8)
I believe the answer is:
fearsome
'formidable' is the definition.
(I've seen this before)
'loud organ remarkable' is the wordplay.
'loud' becomes 'f' (A musical term).
'organ' becomes 'ear' (ear is a kind of organ).
'remarkable' becomes 'some' ('some' can be a synonym of 'remarkable').
'f'+'ear'+'some'='FEARSOME'
(Other definitions for fearsome that I've seen before include "Frightful" , "Daunting" , "Horrendous" , "Terrifying" , "Awful" .)