Criminal case turned on deeply ingrained habit (6,6)
I believe the answer is:
second nature
'deeply ingrained habit' is the definition.
(I've seen this before)
'criminal case turned on' is the wordplay.
'criminal' is an anagram indicator.
'case'+'turned'+'on'='caseturnedon'
'caseturnedon' anagrammed gives 'SECOND NATURE'.
(Other definitions for second nature that I've seen before include "Deeply ingrained habit" , "Almost instinctive behaviour" , "Deeply ingrained tendency" , "it's become instinctive" , "Apparently innate behaviour" .)