During viva, German reflected in a slow and dignified manner (5)
I believe the answer is:
'in a slow and dignified manner' is the definition.
(musical term for in a slow tempo)
'during viva german reflected' is the wordplay.
'during' means one lot of letters goes inside another.
'viva' becomes 'oral' (I've seen this before).
'german' becomes 'g' (abbreviation).
'reflected' shows that the letters should be reversed in order.
'oral' enclosing 'g' is 'ogral'.
'ogral' written backwards gives 'LARGO'.
(Other definitions for largo that I've seen before include "Slow movement in music" , "Slow and dignified, like Handel's composition" , "at stately speed" , "(Of music) broad and slow - Bond villain" , "Musically in slow tempo and dignified style" .)