Disorder requires admission without surgery (7)
I believe the answer is:
entropy
'disorder' is the definition.
(thesaurus)
'admission without surgery' is the wordplay.
'admission' becomes 'entry' (I've seen this before).
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'surgery' becomes 'op' (short for operation).
'entry' enclosing 'op' is 'ENTROPY'.
'requires' is the link.
(Other definitions for entropy that I've seen before include "Thermodynamic quantity" , "lack of order" , "Term in physics" , "Try open (anag) - disorder in the system" , "Physical state" .)