Let the thing a long time with German backing? Exactly! (9)
I believe the answer is:
literatim
'exactly' is the definition.
(thesaurus)
'let the thing a long time with german backing?' is the wordplay.
'let the thing' becomes 'lit' (I can't justify this - if you can you should give a lot more credence to this answer).
'a long time' becomes 'era' (a long period of time).
'with german' becomes 'mit' ('with' in German).
'backing?' says the letters should be written backwards (I've seen 'backing' mean this).
'mit' back-to-front is 'tim'.
'lit'+'era'+'tim'='LITERATIM'
Can you help me to learn more?
(Other definitions for literatim that I've seen before include "showing no change of character" , "one letter at a time" .)