Damage new hat on long-distance race (8)
I believe the answer is:
marathon
'long-distance race' is the definition.
(26 mile race)
'damage new hat on' is the wordplay.
'damage' becomes 'mar' (to mar is to spoil or damage).
'new' is an anagram indicator.
'hat' is an anagram of 'ath'.
'mar'+'ath'+'on'='MARATHON'
(Other definitions for marathon that I've seen before include "no easy task" , "The longest foot-race" , "Pheidippides' starting point" , "Roman hat (anag) - race" , "Long and arduous" .)