It has to be closed to hum perhaps (5)
I believe the answer is:
mouth
'it has to be closed' is the definition.
The definition suggests a singular noun which matches the answer.
'to hum perhaps' is the wordplay.
'perhaps' indicates an anagram.
'to'+'hum'='tohum'
'tohum' anagrammed gives 'MOUTH'.
Can you help me to learn more?
(Other definitions for mouth that I've seen before include "Opening of a bay" , "River part - conversation opener?" , "chops" , "trap" , "River ending" .)