Petty criminal in one clan's quiet (15)
I believe the answer is:
inconsequential
'petty' is the definition.
(thesaurus)
'criminal in one clan's quiet' is the wordplay.
'criminal' is an anagram indicator.
'in'+'one'+'clans'+'quiet'='inoneclansquiet'
'inoneclansquiet' anagrammed gives 'INCONSEQUENTIAL'.
(Other definitions for inconsequential that I've seen before include "Petty" , "Trivial" , "unrelated to the foregoing" .)