Old language the French can put together (5)
I believe the answer is:
latin
'old language' is the definition.
(I've seen this before)
'the french can put together' is the wordplay.
'the french' becomes 'la' ('the' in French).
'can' becomes 'tin' (synonyms).
'put together' means one lot of letters go next to another.
'la'+'tin'='LATIN'
(Other definitions for latin that I've seen before include "Hispanic, for example" , "The language of old Rome" , "Parent language of French,Romanian" , "Language of the Caesars" , "foreign language" .)