Grim-sounding character therefore upset? (4)
I believe the answer is:
ogre
'grim-sounding character' is the definition.
I can't judge whether this defines the answer.
'therefore upset?' is the wordplay.
'therefore' becomes 'ergo' (I've seen this before).
'upset?' shows that the letters should be reversed in order (I've seen 'upset' mean this).
'ergo' written backwards gives 'OGRE'.
Can you help me to learn more?
(Other definitions for ogre that I've seen before include "character in 'Puss in Boots'" , "One making giant strides" , "Giant of folklore, eats humans" , "Bogeyman" , "Fairy tale giant" .)