9 across
Hum
–
to
set
trap
(5)
Hum – to set trap (5)
hints
0
checks
0
hints
The definition is at the end of the clue.
How the clue works
Look at trap — that's your definition, since "trap" is slang for mouth. Then "set" does the work as your anagram indicator, scrambling the letters of hum to (H, U, M, T, O) to give you the answer.