The German youth of the 1950s was faster (6)
I believe the answer is:
dieted
'the 1950s was faster' is the definition.
I can't tell whether this definition defines the answer.
'the german youth' is the wordplay.
'the german' becomes 'die' ('the' in German).
'youth' becomes 'ted' (I can't explain this - if you can you should give a lot more credence to this answer).
'die'+'ted'='DIETED'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for dieted that I've seen before include "Ate sparingly to lose weight" , "Went on strict food regime" , "Followed a food regime" , "Undertook restricted food regime" , "Tried to lose weight" .)