Smashing beginning to Cheltenham racing (8)
I believe the answer is:
crushing
'smashing' is the definition.
(thesaurus)
'beginning to cheltenham racing' is the wordplay.
'beginning to' indicates taking the first letters.
'racing' becomes 'rushing' ('rush' can be a synonym of 'race').
The initial letter of 'cheltenham' is 'c'.
'c'+'rushing'='CRUSHING'
(Other definitions for crushing that I've seen before include "Squeezing together" , "Violently subduing (opposition)" .)