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 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).
(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".)