A leper's dire deterioration (7)
I believe the answer is:
relapse
'deterioration' is the definition.
(thesaurus)
'a leper's dire' is the wordplay.
'dire' indicates an anagram.
'a'+'lepers'='alepers'
'alepers' anagrammed gives 'RELAPSE'.
(Other definitions for relapse that I've seen before include "Pleaser may slip to earlier state" , "Get worse again" , "Fall back into bad health" , "Backsliding" , "Backslide as in health matters" .)