Seat German fellow in black mass (6)
I believe the answer is:
bottom
'seat' is the definition.
(synonyms)
'german fellow in black mass' is the wordplay.
'german fellow' becomes 'otto' (common German name).
'in' indicates putting letters inside.
'black' becomes 'b' (as in pencil classifications).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'b'+'m'='bm'
'otto' going within 'bm' is 'BOTTOM'.
(Other definitions for bottom that I've seen before include "foot" , "Behind" , "seat" , "Low point" , "Lowest part or underside" .)