Races off to boast - that's frightening (9)
I believe the answer is:
scarecrow
'that's frightening' is the definition.
'races off to boast' is the wordplay.
'off' indicates an anagram.
'to boast' becomes 'crow' (to crow about something is to boast about it).
'races' with letters rearranged gives 'scare'.
'scare'+'crow'='SCARECROW'
(Other definitions for scarecrow that I've seen before include "Ragged figure in field" , "one deters" , "Figure to deter birds" , "Badly dressed person" , "Untidy person" .)