Awfully bad line, message ultimately unsubstantiated (8)
I believe the answer is:
deniable
'unsubstantiated' is the definition.
(something unsubstantiated can still be denied)
'awfully bad line message ultimately' is the wordplay.
'awfully' indicates an anagram.
'ultimately' suggests the final letters.
The final letter of 'message' is 'e'.
'bad'+'line'='badline'
'badline' is an anagram of 'deniabl'.
'deniabl'+'e'='DENIABLE'
(Other definitions for deniable that I've seen before include "open to question" , "Able to be gainsaid" .)