A number entered here — vital to move quietly (6)
I believe the answer is:
keypad
'a number entered here' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'vital to move quietly' is the wordplay.
'vital' becomes 'key' (I've seen this before).
'to' means one lot of letters go next to another.
'move quietly' becomes 'pad' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'key'+'pad'='KEYPAD'
Can you help me to learn more?
(Other definitions for keypad that I've seen before include "Phone's button array" , "Device for data entry" , "Typing area on an electronic device" , "Data input device for computers etc" , "where characters get depressed" .)