Once ordered an ice cream (4)
I believe the answer is:
cone
'an ice cream' is the definition.
(cone can refer to an ice cream cone)
'once ordered' is the wordplay.
'ordered' indicates anagramming the letters.
'once' with letters rearranged gives 'CONE'.
(Other definitions for cone that I've seen before include "standing in middle of motorway" , "Tapering shape" , "Geometrical figure (used on roads!)" , "Roadworks marker" , "Motorway menace" .)