Old language the French preserve (5)
I believe the answer is:
latin
'old language' is the definition.
(Latin is a dead language)
'the french preserve' is the wordplay.
'the french' becomes 'la' ('the' in French).
'preserve' becomes 'tin' (tinning is a kind of preserving).
'la'+'tin'='LATIN'
(Other definitions for latin that I've seen before include "Parent language of French,Romanian" , "Language of Cicero" , "ancient tongue" , "Language of ancient Rome" , "Shakespeare's was small, according to Jonson" .)