Bar accommodating one hundred runners at the start (6)
I believe the answer is:
police
'runners at the start' is the definition.
(policemen were historically called runners)
'bar accommodating one hundred' is the wordplay.
'bar' becomes 'pole' ().
'accommodating' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'hundred' becomes 'c' (Roman numeral).
'i'+'c'='ic'
'pole' placed around 'ic' is 'POLICE'.
(Other definitions for police that I've seen before include "Gardai" , "Limbs of the law" , "Constabulary" , "Long arm of the law" , "Civil force" .)