Looking ruddy angry! (3)
I believe the answer is:
red
'angry' is the definition.
(I've seen this in other clues)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for red that I've seen before include "River, its source in the US, flowing north to empty into L Winnipeg" , "A colourful communist" , "Colour for a blusher" , "Colour of blood" , "'If you see it, perhaps you're angry (3)'" .)