Get worse - get worse ere slap (7)
I believe the answer is:
relapse
'get worse' is the definition.
(synonyms)
'worse ere slap' is the wordplay.
'worse' indicates anagramming the letters (a worse or damaged version of the letters).
'ere'+'slap'='ereslap'
'ereslap' anagrammed gives 'RELAPSE'.
'get' is the link.
(Other definitions for relapse that I've seen before include "fall into sin" , "Get into bad habits again" , "Deterioration in health" , "Deteriorate in health" , "causing setback to recovery" .)