Insists chief isn't a criminal (9)
I believe the answer is:
maintains
'insists' is the definition.
(maintaining is a kind of insisting)
'chief isn't a criminal' is the wordplay.
'chief' becomes 'main' (I've seen this before).
'criminal' is an anagram indicator.
'isnt'+'a'='isnta'
'isnta' anagrammed gives 'tains'.
'main'+'tains'='MAINTAINS'
(Other definitions for maintains that I've seen before include "Looks after" , "Keeps" .)