Most modern part of the US, regularly a vital part (5,2,3,3)
I believe the answer is:
state of the art
'most modern' is the definition.
(state of the art means with the latest technology)
'part of the us regularly a vital part' is the wordplay.
'part of the us' becomes 'state' (part of the United States of America).
'regularly' becomes 'oft' (often).
'a vital part' becomes 'heart' (vital organ).
'state'+'oft'+'heart'='STATE-OF-THE-ART'
(Other definitions for state of the art that I've seen before include "incorporating the most up-to-date features" , "Most hi-tech" , "Highest level of development at the present time" , "Technologically most recent" , "concern of museum conservationist?" .)