All upset about the wreck that was fatal (6)
I believe the answer is:
lethal
'fatal' is the definition.
(similar in meaning)
'all upset about the wreck' is the wordplay.
'upset' indicates an anagram.
'about' means one lot of letters goes inside another.
'wreck' is an anagram indicator.
'all' anagrammed gives 'lal'.
'the' anagrammed gives 'eth'.
'lal' placed around 'eth' is 'LETHAL'.
'that was' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for lethal that I've seen before include "Vicious" , "describing a killer?" , "bringing about the end?" , "Capable of causing death" , "All the change can be deadly." .)