Yells and races off without thinking (10)
I believe the answer is:
carelessly
'without thinking' is the definition.
'yells and races off' is the wordplay.
'and' says to put letters next to each other.
'off' indicates an anagram.
'yells' put after 'races' is 'racesyells'.
'racesyells' is an anagram of 'CARELESSLY'.
(Other definitions for carelessly that I've seen before include "Incautiously" , "Without a thought" , "Casually" , "Without concern" , "without proper attention" .)