SAS covering miles on horse inflict defeat (5)
I believe the answer is:
smash
'defeat' is the definition.
(I know that smash is a type of demolish)
'sas covering miles on horse' is the wordplay.
'covering' means one lot of letters goes inside another.
'miles' becomes 'm' (common abbreviation).
'on' says to put letters next to each other.
'horse' becomes 'h' (synonyms).
'sas' going around 'm' is 'smas'.
'smas'+'h'='SMASH'
'inflict' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for smash that I've seen before include "- - and grab" , "Break like a glass or cup" , "Wreck" , "Ruin; big (success)" , "Shatter, break up" .)