Initially electronic tag raised barrier (4)
I believe the answer is:
gate
'barrier' is the definition.
(I know that gate is a type of movable barrier)
'initially electronic tag raised' is the wordplay.
'initially' indicates taking the first letters.
'raised' says the letters should be written in reverse (going up in a down clue).
The first letter of 'electronic' is 'e'.
'e'+'tag'='etag'
'etag' back-to-front is 'GATE'.
(Other definitions for gate that I've seen before include "Plane-boarding point" , "Facility in airport" , "Airport boarding point" , "It may be barred" , "Attendance at a sporting event" .)