Certain to be clean isn't bad (13)
I believe the answer is:
incontestable
'certain' is the definition.
(thesaurus)
'to be clean isn't bad' is the wordplay.
'bad' is an anagram indicator.
'to'+'be'+'clean'+'isnt'='tobecleanisnt'
'tobecleanisnt' is an anagram of 'INCONTESTABLE'.
(Other definitions for incontestable that I've seen before include "Without doubt" , "sure" , "Beyond dispute" .)