Security on door vital at flat (6)
I believe the answer is:
keypad
'security on door' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'vital at flat' is the wordplay.
'vital' becomes 'key' (key can mean important).
'at' means one lot of letters go next to another.
'flat' becomes 'pad' (a person's home).
'key'+'pad'='KEYPAD'
Can you help me to learn more?
(Other definitions for keypad that I've seen before include "Feature used to operate a portable electronic device" , "What might help you calculate" , "Typing area on an electronic device" , "Mobile phone provides this" , "where one may have pressing needs" .)