In favour of a piece of bread with egg on (4)
I believe the answer is:
prod
'egg on' is the definition.
(synonyms)
'in favour of a piece of bread' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'a piece of bread' becomes 'd' (I can't explain this - if you can you should give a lot more credence to this answer).
'pro'+'d'='PROD'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for prod that I've seen before include "Urge or stimulate" , "Poke; reminder" , "Poke with a stick" , "Goad, stimulate into action" , "Poke with forefinger, stimulate" .)