Important piece of music (12)
I believe the answer is:
instrumental
I believe this clue is a triple definition!
'important' is the first definition.
(I've seen this before)
'piece' is the second definition.
(I have seen 'piece of music' mean 'instrumental' so perhaps 'piece' could also mean 'instrumental')
'of' acts as a link.
'music' is the third definition.
(I've seen this before)
(Other definitions for instrumental that I've seen before include "Like a musical piece - it's of help" , "Piece of music without voices" , "Having some responsibility - not vocal" , "doing without singers" , "Important" .)