Boring noise that comes with percussion instrument (7)
I believe the answer is:
humdrum
'boring' is the definition.
(thesaurus)
'noise that comes with percussion instrument' is the wordplay.
'noise' becomes 'hum' (humming is a kind of noising).
'that comes with' is a charade indicator (letters next to each other).
'percussion instrument' becomes 'drum' (drum is a kind of percussion instrument).
'hum'+'drum'='HUMDRUM'
(Other definitions for humdrum that I've seen before include "without incident" , "Run of the mill" , "Routine and monotonous" , "Monotonous; commonplace" , "Run-of-the-mill" .)