10 across
Red
four
from
sevens
or
eights
(4)
Red four from sevens or eights (4)
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The definition is at the beginning of the clue.
How the clue works
See how "four from sevens or eights" is nudging you to just take four letters from that run of words - well, sevenS OR Eights, read straight across, spells out SORE hiding right there in plain sight. And that gives you the meaning too, since Red is just another way of saying inflamed or angry, which is the definition.