Say, good source of protein (3)
I believe the answer is:
egg
'source of protein' is the definition.
'say good' is the wordplay.
'say' becomes 'eg'.
'good' becomes 'g' (abbreviation).
'eg'+'g'='EGG'
(Other definitions for egg that I've seen before include "nit? Just a bit!" , "Poultry product" , "On the face, it's embarrassing" , "Bomb or mine (sl.)" , "Food in a shell" .)