Crossword Genius

Upset and feverish, taking a month off (4)

Ross

I believe the answer is:

rile

'upset' is the definition.
(to rile is to annoy or upset)

I don't understand how the remainder of the clue works.

(Other definitions for rile that I've seen before include "Make angry" , "Bother" , "Informally, annoy or irritate" , "Lire (anag)" , "Stir up anger" .)

I've seen this clue in The Times.
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