Protein sources - for Romans they are found in lovage (4)
I believe the answer is:
eggs
'romans they are found in lovage' is the definition.
The definition suggests an adverb but the answer is not.
'protein sources' is the wordplay.
'protein' becomes 'egg' (source of protein).
'sources' becomes 's' (this could be a standard abbreviation of which I'm unaware).
'egg'+'s'='EGGS'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for eggs that I've seen before include "Ovoid objects" , "Bacon's partners" , "Don't teach your grandmother to suck --" , "Roe" , "Incites - farm produce" .)