For example British beef is a source of protein (3,3)
I believe the answer is:
egg box
'a source of protein' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'for example british beef' is the wordplay.
'for example' becomes 'eg' ('e.g.' can be a synonym of 'for example').
'british' becomes 'gb' (abbreviation for Great Britain).
'beef' becomes 'ox'.
'eg'+'gb'+'ox'='EGG-BOX'
'is' is the link.
Can you help me to learn more?
(Other definitions for egg box that I've seen before include "Food container" , "Partitioned container from deli" .)