Politician in middle, one running initiative (8)
I believe the answer is:
gumption
'initiative' is the definition.
(gumption can mean initiative or ingenuity)
'politician in middle one running' is the wordplay.
'politician' becomes 'MP' (Member of Parliament).
'in' means one lot of letters goes inside another.
'middle' becomes 'gut' (a person's middle).
'one' becomes 'i' (Roman numeral).
'running' becomes 'on' (eg when a machine is running it is on).
'mp' placed within 'gut' is 'gumpt'.
'gumpt'+'i'+'on'='GUMPTION'
(Other definitions for gumption that I've seen before include "To impugn (anag) - common sense" , "Informal drive and initiative" , "Nous" , "Sense; courage" , "Informal common sense or initiative" .)