Boring noise made by mouth on instrument (7)
I believe the answer is:
Here is my best explanation:
'boring' is the definition.
'noise made by mouth on instrument' is the subsidiary indication.
'noise made by mouth' becomes 'hum'.
'on' is a charade indicator (letters next to each other).
'instrument' becomes 'drum' (drum is a kind of instrument).
(Other definitions for humdrum that I've seen before include "Everyday, tedious", "Dull, monotonous", "dull and uninteresting", "Boringly monotonous", "Run of the mill", "Dull (of existence)", "Common place", "Unexciting", "Tediously repetitous or lacking in variety" and "Commonplace, dull".)