Casper travels a long way (6)
I believe the answer is:
parsec
'a long way' is the definition.
(I've seen this before)
'casper travels' is the wordplay.
'travels' indicates an anagram.
'casper' anagrammed gives 'PARSEC'.
(Other definitions for parsec that I've seen before include "Three+ light-years" , "fair distance" , "Unit of astronomical distance - scrape (anag)" , "Unit of distance in astronomy" , "unit for measuring distances" .)