Part company with politician, one leading national organisation (9)
I believe the answer is:
component
'part' is the definition.
(component is a kind of part)
'company with politician one leading national organisation' is the wordplay.
'company' becomes 'co' (abbreviation).
'with' says to put letters next to each other.
'politician' becomes 'mp'.
'leading' says to put letters next to each other.
'national organisation' becomes 'nt' (I've seen this in other clues).
'co'+'mp'+'one'+'nt'='COMPONENT'
(Other definitions for component that I've seen before include "Constituent part" , "Part of a whole" , "core part" , "Element of a larger whole" , "Element of assembly, elec. circuit" .)