Toast, one round with topping of egg on (7)
I believe the answer is:
cheerio
'toast' is the definition.
(I've seen this before)
'one round with topping of egg on' is the wordplay.
'one round with topping of' becomes 'io' (I am not sure about this - if you are sure you should believe this answer much more).
'egg' becomes 'cheer' (both can mean to encourage someone on).
'on' says to put letters next to each other.
'io' put after 'cheer' is 'CHEERIO'.
Can you help me to learn more?
(Other definitions for cheerio that I've seen before include "Bye-bye" , "Final message" , "for so long!" , "A hearty farewell, begins with applause" , "Informal goodbye, sounds blithe" .)