In French describing old age (3)
I believe the answer is:
eon
'age' is the definition.
(I know that eon is a type of age)
'in french describing old' is the wordplay.
'in french' becomes 'en' ('in' in French).
'describing' indicates putting letters inside (describe can mean to form the outline of).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'en' placed around 'o' is 'EON'.
(Other definitions for eon that I've seen before include "Long period of time" , "Prolonged period" , "plenty of time" , "Age, epoch" , "Major division of geological time" .)