Careless pugilist punching explorer in the dark (8)
I believe the answer is:
cavalier
'careless' is the definition.
(I know that careless can be written as cavalier)
'pugilist punching explorer in the dark' is the wordplay.
'pugilist' becomes 'Ali' (Muhammad Ali - a pugilist is a boxer).
'punching' is an insertion indicator (some letters punch a hole in others).
'explorer in the dark' becomes 'caver' (someone who explores caves).
'ali' placed within 'caver' is 'CAVALIER'.
(Other definitions for cavalier that I've seen before include "Roundhead's adversary" , "... King Charles, canine breed" , "Offhand, careless" , "Courtly gent - laughing in painting" , "A vile car is offhand" .)