Diligent cousins notice changes (13)
I believe the answer is:
conscientious
'diligent' is the definition.
(thesaurus)
'cousins notice changes' is the wordplay.
'changes' is an anagram indicator.
'cousins'+'notice'='cousinsnotice'
'cousinsnotice' with letters rearranged gives 'CONSCIENTIOUS'.
(Other definitions for conscientious that I've seen before include "Taking great pains" , "thorough" , "Scrupulous" , "Wanting to do right" , "Diligent" .)