Boring noise made by mouth on instrument (7)
I believe the answer is:
humdrum
'boring' is the definition.
(thesaurus)
'noise made by mouth on instrument' is the wordplay.
'noise made by mouth' becomes 'hum'.
'on' means one lot of letters go next to another.
'instrument' becomes 'drum' (drum is a kind of instrument).
'hum'+'drum'='HUMDRUM'
(Other definitions for humdrum that I've seen before include "Run-of-the-mill" , "Workaday" , "Droning sounds" , "Boringly monotonous" , "Dull, monotonous" .)