Part of entrance security vital to flat (6)
I believe the answer is:
keypad
'part of entrance security' is the definition.
(as in a keypad on a door)
'vital to flat' is the wordplay.
'vital' becomes 'key' (I've seen this in other clues).
'to flat' becomes 'pad' (a person's home).
'key'+'pad'='KEYPAD'
(Other definitions for keypad that I've seen before include "Mobile phone provides this" , "Typing area on an electronic device" , "Device for data entry" , "Feature used to operate a portable electronic device" , "Small unit with push buttons" .)