German linguist forbidding using repeated letter at the end (5)
I believe the answer is:
grimm
'german linguist' is the definition.
(Jacob Grimm)
'forbidding using repeated letter at the end' is the wordplay.
'forbidding using' becomes 'gri' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'repeated' says to double the letters.
'letter' becomes 'm'.
'at the end' is a charade indicator (letters next to each other).
'm' duplicated is 'mm'.
'gri'+'mm'='GRIMM'
(Other definitions for grimm that I've seen before include "The Brothers ..... wrote fairytales" , "Brothers who told stories" , "Fairy tale telling brother" , "German folklorist" , "taletellers" .)