Unavoidable day matron goes mad (9)
I believe the answer is:
mandatory
'unavoidable' is the definition.
'day matron goes mad' is the wordplay.
'goes mad' indicates an anagram.
'day'+'matron'='daymatron'
'daymatron' anagrammed gives 'MANDATORY'.
(Other definitions for mandatory that I've seen before include "Not optional" , "Binding" , "required" , "allowing no option" , "Obligatory" .)