Small toy, one that hums? (7)
I believe the answer is:
stinker
'one that hums?' is the definition.
(hum can mean to stink or smell)
'small toy' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'toy' becomes 'tinker' (I've seen this in another clue).
's'+'tinker'='STINKER'
(Other definitions for stinker that I've seen before include "Terrible job" , "Unpleasant person - difficult task" , "Very hard task" , "no easy thing" , "Difficult problem" .)