Ultimately, US city street likely empty (6)
I believe the answer is:
lastly
'ultimately' is the definition.
(thesaurus)
'us city street likely empty' is the wordplay.
'us city' becomes 'LA' (abbreviation for Los Angeles).
'street' becomes 'st' (abbreviation).
'empty' means to remove the middle letters.
'likely' with its middle removed is 'ly'.
'la'+'st'+'ly'='LASTLY'
(Other definitions for lastly that I've seen before include "In the end" , "In conclusion" , "Finally, to conclude with" .)