Song serious about Germans' leader (5)
I believe the answer is:
dirge
'song' is the definition.
(I know that dirge is a type of song)
'serious about germans' leader' is the wordplay.
'serious' becomes 'dire' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'leader' indicates taking the first letters.
The initial letter of 'germans' is 'g'.
'dire' enclosing 'g' is 'DIRGE'.
(Other definitions for dirge that I've seen before include "Funeral music" , "Mournful music" , "No joy in this music" , "Sombre music" , "Funeral hymn" .)