Seat German chap in black mass (6)

I believe the answer is:

bottom

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'seat german' is the definition.
'bottom' can be an answer for 'seat' (I've seen this before). I'm not certain of the 'german' bit.

'chap in black mass' is the wordplay.
'chap' becomes 'otto' ().
'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' inserted into 'bm' is 'BOTTOM'.

Can you help me to learn more?

(Other definitions for bottom that I've seen before include "Lowest point - weaver" , "Quark's flavour" , "MND weaver" , "Mechanical" , "Lowest of the low" .)

I've seen this clue in The Sun.

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