It screens unwelcome visitors in two ways (5)
I believe the answer is:
radar
'it screens unwelcome' is the definition.
The definition suggests a singular noun which matches the answer.
'visitors in two ways' is the wordplay.
'two ways' means two substitutions for 'way'.
'visitors' becomes 'ada' (I am not sure about this - if you are sure you should believe this answer much more).
'in' is an insertion indicator.
'way' becomes 'R' (abbreviation for right).
'way' becomes 'R' (abbreviation for right).
'r'+'r'='rr'
'ada' inserted into 'rr' is 'RADAR'.
Can you help me to learn more?
(Other definitions for radar that I've seen before include "System for detecting things using radio waves" , "System for detecting objects with radio waves" , "A help in locating" , "Navigational aid" , "System that detects and locates distant objects" .)