One not serving polite Scotsman? (8)
I believe the answer is:
civilian
'one not serving' is the definition.
(as opposed to a soldier)
'polite scotsman?' is the wordplay.
'polite' becomes 'civil' ('civil' can be a synonym of 'polite').
'scotsman?' becomes 'ian' (I've seen this before).
'civil'+'ian'='CIVILIAN'
(Other definitions for civilian that I've seen before include "out of service" , "One is not a member of the police or armed forces" , "Nonmilitary person" , "Non-combatant" , "Layman found in a civil way" .)