Reported effect of traffic jam in Paris? Sad (6)

I believe the answer is:

rueful

'sad' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for rueful that I've seen before include "Regretful" , "In a sorry state!" , "'Contrite, remorseful (6)'" , "Penitent" , "Regretting" .)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app