Thoughtless ace rival shot (8)
I believe the answer is:
cavalier
'thoughtless' is the definition.
(I've seen this before)
'ace rival shot' is the wordplay.
'shot' indicates an anagram.
'ace'+'rival'='acerival'
'acerival' with letters rearranged gives 'CAVALIER'.
(Other definitions for cavalier that I've seen before include "Shout out noisily" , "Civil War participant" , "English Civil War royalist" , "... King Charles, spaniel" , "Offhand - Charles I supporter" .)