Assist dour characters, agitated because it's absolutely terrible (10)
I believe the answer is:
disastrous
'terrible' is the definition.
(thesaurus)
'assist dour characters' is the wordplay.
'characters' indicates anagramming the letters.
'assist'+'dour'='assistdour'
'assistdour' is an anagram of 'DISASTROUS'.
'agitated because it's absolutely' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for disastrous that I've seen before include "Causing great damage" , "Highly unsuccessful" , "Catastrophic" , "Dire" .)