Pearls cast before swine finally to deteriorate (7)
I believe the answer is:
relapse
'to deteriorate' is the definition.
(as in a relapse of a medical condition)
'pearls cast before swine finally' is the wordplay.
'cast' indicates anagramming the letters (letters are dispersed or cast around).
'before' means one lot of letters go next to another.
'finally' suggests the final letters.
The final letter of 'swine' is 'e'.
'pearls' with letters rearranged gives 'relaps'.
'relaps'+'e'='RELAPSE'
(Other definitions for relapse that I've seen before include "Get worse again" , "setback in health terms" , "Downturn in health (after improvement)" , "Fall ill again after rallying" , "Degenerate like pleaser" .)