Failure to accept looking old may be reversed with injection of article (6)
I believe the answer is:
denial
'failure' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for denial that I've seen before include "Refusal to admit painful truths" , "Repudiation" , "Refusal to acknowledge" , "Daniel (anag.)" , "Rejection or refusal" .)