Trolley contains bun (4)
I believe the answer is:
roll
'bun' is the definition.
('roll' can be a synonym of 'bun')
'trolley contains' is the wordplay.
'contains' indicates a hidden word.
'ROLL' is hidden in the letters of 'trolley'.
(Other definitions for roll that I've seen before include "Cylinder (of paper etc)" , "movement of ship" , "Official list; cylinder" , "Move like a ball" , "Sound of a drum or a list of names" .)