Pearls cast before swine ultimately causes setback (7)
I believe the answer is:
relapse
'setback' is the definition.
(thesaurus)
'pearls cast before swine ultimately' is the wordplay.
'cast' indicates anagramming the letters.
'before' means one lot of letters go next to another.
'ultimately' indicates one should take the final letters.
The last letter of 'swine' is 'e'.
'pearls' anagrammed gives 'relaps'.
'relaps'+'e'='RELAPSE'
'causes' is the link.
(Other definitions for relapse that I've seen before include "Get worse again" , "Deterioration in health" , "Fall back into sickness" , "Deterioration after period of improvement" , "Fail to maintain progress" .)