Awful delay after day in terminal (6)
I believe the answer is:
deadly
'terminal' is the definition.
(thesaurus)
'awful delay after day' is the wordplay.
'awful' indicates an anagram.
'after' says to put letters next to each other.
'day' becomes 'd'.
'delay' is an anagram of 'eadly'.
'eadly' put after 'd' is 'DEADLY'.
'in' acts as a link.
(Other definitions for deadly that I've seen before include "Lethal, capable of causing fatal injury" , "Lethal or mortal" , "Lethal, could be fatal" , "Lethal, capable of causing fatality" , "Lethal, capable of killing" .)