Barrier beginning to worry everyone (4)
I believe the answer is:
wall
'barrier' is the definition.
(a wall acts as a barrier)
'beginning to worry everyone' is the wordplay.
'beginning to' says to take the initial letters.
'everyone' becomes 'all' (all people).
The first letter of 'worry' is 'w'.
'w'+'all'='WALL'
(Other definitions for wall that I've seen before include "Built barrier" , "Side of building/room" , "See 1" , "Solid barrier" , "mechanical part" .)