Prompt restraint by the German retains millions (8)
I believe the answer is:
reminder
'prompt' is the definition.
(I've seen this before)
'restraint by the german retains millions' is the wordplay.
'restraint by' becomes 'rein' (I can't justify this - if you can you should give a lot more credence to this answer).
'the german' becomes 'der' ('the' in German).
'retains' means one lot of letters goes inside another.
'millions' becomes 'm' (abbreviation).
'rein'+'der'='reinder'
'reinder' placed around 'm' is 'REMINDER'.
Can you help me to learn more?
(Other definitions for reminder that I've seen before include "Aide-memoire" , "note" , "Memento" , "Memory jogger" , "Knot in handkerchief?" .)