Lord Muck initially in list supporting aristocrat (8)
I believe the answer is:
nobleman
'lord' is the definition.
('nobleman' can be a synonym of 'lord')
'muck initially in list supporting aristocrat' is the wordplay.
'initially' says to take the initial letters.
'in list' becomes 'an' (I am not sure about this - if you are sure you should believe this answer much more).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'aristocrat' becomes 'noble' (noble is a kind of aristocrat).
The initial letter of 'muck' is 'm'.
'm'+'an'='man'
'man' put after 'noble' is 'NOBLEMAN'.
Can you help me to learn more?
(Other definitions for nobleman that I've seen before include "I have a title" , "member of Upper House" , "Male aristocrat" , "Member of peerage" , "Member of the aristocracy" .)