French student's new number (6)
I believe the answer is:
eleven
'number' is the definition.
(eleven is a kind of number)
'french student's new' is the wordplay.
'french student' becomes 'eleve' ('student' in French).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'eleve'+'n'='ELEVEN'
(Other definitions for eleven that I've seen before include "XI as Roman numeral" , "Cricket 18" , "Cricketing side" , "Odd number for a team" , "sports team?" .)