Lunatic on treatment without interruption (9)
I believe the answer is:
continual
'without interruption' is the definition.
(I've seen this before)
'lunatic on treatment' is the wordplay.
'treatment' is an anagram indicator (letters treated or processed differently).
'lunatic'+'on'='lunaticon'
'lunaticon' is an anagram of 'CONTINUAL'.
(Other definitions for continual that I've seen before include "Non-stop" , "Constantly occurring" , "Happening all the time" , "Incessant" , "Without interruption" .)