Old tongue preserved in gelatine (5)
I believe the answer is:
latin
'old tongue' is the definition.
'preserved in gelatine' is the wordplay.
'preserved in' indicates a hidden word.
'LATIN' is hidden in the letters of 'gelatine'.
(Other definitions for latin that I've seen before include "language class" , "Romantic language" , "Classical language" , "Learning it 'an honour' (Churchill)" , "Language of ancient Rome" .)