Mum runs over solemn German nobleman (8)
I believe the answer is:
margrave
'german nobleman' is the definition.
(I've seen this before)
'mum runs over solemn' is the wordplay.
'mum' becomes 'ma' (both can mean mother).
'runs' becomes 'r' (cricket abbreviation).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'solemn' becomes 'grave' (grave can mean solemn or serious).
'ma'+'r'+'grave'='MARGRAVE'
(Other definitions for margrave that I've seen before include "German noble" , "German nobleman" , "More than count" .)