Soluble protein, say, returning to old tongue (7)
I believe the answer is:
gelatin
'soluble protein' is the definition.
'gelatin' can be an answer for 'protein' (gelatin is a kind of protein). I'm not sure about the 'soluble' bit.
'say returning to old tongue' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'returning' shows that the letters should be reversed in order.
'to old tongue' becomes 'latin' (Latin is an ancient language).
'eg' back-to-front is 'ge'.
'ge'+'latin'='GELATIN'
Can you help me to learn more?
(Other definitions for gelatin that I've seen before include "Glue made from animal extracts" , "Substance used in glue and cooking" , "Tag line (anag.); glue" , "Setter" , "Food/glue jelly" .)