Derided regressive content that's truly appalling (4)
I believe the answer is:
dire
'truly appalling' is the definition.
'dire' can be an answer for 'appalling' (I've seen this before). I am unsure of the 'truly' bit.
'derided regressive content' is the wordplay.
'content' indicates the answer is hidden within the clue (letters contained within the clue).
'regressive' says the letters should be written in reverse (to regress is to go backwards).
'erid' is hidden within 'derided'.
'erid' in reverse letter order is 'DIRE'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for dire that I've seen before include "Portentous" , "Disastrous" , "Cataclysmic" , "Catastrophic" , "Ride off in dreadful way" .)