Nowadays, speech is a habit (9)
I believe the answer is:
addiction
'is a habit' is the definition.
(thesaurus)
'nowadays speech' is the wordplay.
'nowadays' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'speech' becomes 'diction' (diction is someone's way of speaking).
'ad'+'diction'='ADDICTION'
(Other definitions for addiction that I've seen before include "The origin of dependence" , "Compulsive habit" , "Obsession" , "Drug-habit" , "Bug one's caught" .)