Work (up) (4)
'work up' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rile that I've seen before include "Bother" , "bug" , "Make angry" , "Provoke" , "Irk" .)
(Other definitions for anger that I've seen before include "Nettle" , "Great annoyance" , "exasperation" , "Deadly sin" , "Hostile feeling" .)