The most common and most likely answer to this crossword clue is the 13 letter word CONTRADICTION
C
O
N
T
R
A
D
I
C
T
I
O
N
More clues leading to the result CONTRADICTION
- Police officer probing one element of Labour opposition
- Conflict of meaning I’d turned up in abridgement
- Detective involved in throttling gives
- Logical flaw I had reversed in short form
- Clash with prisoner caught during convention
- Disagreement when Tory party convention welcomes leader of coalition
- Denial by criminal caught breaking rule
- Inconsistency in speech by the opposing side
Buddy explains!

The straight part
We know that "Denial" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case denial = contradiction.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CONTRADICTION mean?
CONTRADICTION noun- The act of contradicting.
- A statement that contradicts itself, i.e., a statement that makes a claim that the same thing is true and that it is false at the same time and in the same senses of the terms.
- A logical inconsistency among two or more elements or propositions.
- A proposition that is false for all values of its variables.