Overrule patient loathing leaving hospital (10)
I believe the answer is:
invalidate
'overrule' is the definition.
(thesaurus)
'patient loathing leaving hospital' is the wordplay.
'patient' becomes 'invalid' (I've seen this before**).
'loathing' becomes 'hate' (I've seen this before).
'leaving' suggests deleting specific letters.
'hospital' becomes 'h'.
'hate' with 'h' taken out is 'ate'.
'invalid'+'ate'='INVALIDATE'
(Other definitions for invalidate that I've seen before include "Disprove" , "End effectiveness" , "Cancel" , "Make nothing of" , "Nullify" .)