I had one order in the vernacular (5)
I believe the answer is:
idiom
'the vernacular' is the definition.
(thesaurus)
'i had one order' is the wordplay.
'i had' becomes 'id' (I'd).
'one' becomes 'i' (Roman numeral).
'order' becomes 'om' (order of merit).
'id'+'i'+'om'='IDIOM'
'in' is the link.
(Other definitions for idiom that I've seen before include "particular phrase" , "Word usage peculiar to a specific group" , "Local style of expression" , "Language peculiar to a particular group" , "Figure of speech" .)