Outburst from one stuck in traffic (6)
I believe the answer is:
tirade
'outburst' is the definition.
(thesaurus)
'one stuck in traffic' is the wordplay.
'one' becomes 'i' (Roman numeral).
'stuck in' means one lot of letters goes inside another.
'traffic' becomes 'trade' (I've seen this before).
'i' put inside 'trade' is 'TIRADE'.
'from' acts as a link.
(Other definitions for tirade that I've seen before include "Verbal broadside" , "Inclination; fish" , "Torrent of abuse" , "Blast of criticism" , "String of invective" .)