Ailment's best treated ringing hospital foundation (13)
I believe the answer is:
establishment
'foundation' is the definition.
(thesaurus)
'ailment's best treated ringing hospital' is the wordplay.
'treated' is an anagram indicator.
'ringing' is an insertion indicator.
'hospital' becomes 'h'.
'ailments'+'best'='ailmentsbest'
'ailmentsbest' anagrammed gives 'establisment'.
'establisment' going around 'h' is 'ESTABLISHMENT'.
(Other definitions for establishment that I've seen before include "Act of setting up" , "system" , "Institutionalized authority" , "Those in power" , "Institution; (the) ruling classes" .)