Motion principally marking leaderless aristocracy (8)
I believe the answer is:
mobility
'motion principally' is the definition.
'mobility' can be an answer for 'motion' (thesaurus). I am not sure about the 'principally' bit.
'marking leaderless aristocracy' is the wordplay.
'marking' becomes 'm' (this could be a standard abbreviation which I don't know about).
'leaderless' means to remove the first letter.
'aristocracy' becomes 'nobility' (synonyms).
'nobility' with its first letter taken off is 'obility'.
'm'+'obility'='MOBILITY'
Can you help me to learn more?
(Other definitions for mobility that I've seen before include "Freedom to get around" , "Freedom of movement" , "Locomotion" .)