Loose rock needs new barrier (6)
I believe the answer is:
screen
'barrier' is the definition.
(screen is a kind of barrier)
'loose rock needs new' is the wordplay.
'loose rock' becomes 'scree' (I've seen this before).
'needs' means one lot of letters go next to another.
'new' becomes 'n'.
'scree'+'n'='SCREEN'
(Other definitions for screen that I've seen before include "TV surface" , "Partition; monitor" , "A film is projected onto it" , "Examine methodically" , "Sheltering row of trees" .)