Anger at the upset about a bad heart (4)
I believe the answer is:
heat
'anger' is the definition.
(I've seen this in another clue)
'the upset about a bad heart' is the wordplay.
'upset' indicates an anagram.
'about' means one lot of letters goes inside another.
'heart' says to take the centre.
The middle of 'bad' is 'a'.
'the' is an anagram of 'het'.
'het' enclosing 'a' is 'HEAT'.
'at' acts as a link.
(Other definitions for heat that I've seen before include "brings out a sweat?" , "Hotness" , "Pressure" , "Energy that causes a rise in temperature" , "Toast" .)