You can eat a third of one in a bus (4)
I believe the answer is:
buns
'you can' is the definition.
'buns' can be an answer for 'can' (synonyms). I am unsure of the 'you' bit.
'eat a third of one in a bus' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for buns that I've seen before include "Small rolls" , "Food items" , "Cakes" .)