To banish a coster is dreadful (9)
I believe the answer is:
ostracise
'to banish' is the definition.
(I've seen this before)
'a coster is dreadful' is the wordplay.
'dreadful' indicates an anagram.
'a'+'coster'+'is'='acosteris'
'acosteris' anagrammed gives 'OSTRACISE'.
(Other definitions for ostracise that I've seen before include "Shun, exclude" , "Drive out" , "Blackball, send to Coventry" , "I, Socrates may exclude from group" , "Socrates and I are made to exclude from society" .)