Once changed traffic marker (4)
I believe the answer is:
cone
'traffic marker' is the definition.
(I've seen this in another clue)
'once changed' is the wordplay.
'changed' is an anagram indicator.
'once' is an anagram of 'CONE'.
(Other definitions for cone that I've seen before include "Geometric solid - fruit of pine" , "Figure (for traffic control?)" , "may obstruct lane" , "Movable plastic bollard" , "it may hold ice-cream" .)