Deep-seated; persistent (7)
I believe the answer is:
chronic
This clue is a double definition.
'deep-seated' is the first definition.
(thesaurus)
'persistent' is the second definition.
(a chronic condition is a persistent one)
(Other definitions for chronic that I've seen before include "pants" , "(Of illness) lengthy; very bad (colloq.)" , "Of illness, prolonged or constantly recurring" , "Very serious" , "Not acute" .)