For example, good source of protein (3)
I believe the answer is:
egg
'source of protein' is the definition.
'for example good' is the wordplay.
'for example' becomes 'e.g.' (short for 'exempli gratia', 'for example' in Latin).
'good' becomes 'g' (abbreviation).
'eg'+'g'='EGG'
(Other definitions for egg that I've seen before include "Food from a hen" , "Bird's first home" , "Over-easy breakfast item?" , "Oval item" , "It comes before or after the chicken?" .)