The most common and most likely answer to this crossword clue is the 13 letter word INCONTESTABLE
I
N
C
O
N
T
E
S
T
A
B
L
E
More clues leading to the result INCONTESTABLE
- Certain moves not nice - steady
- Absolutely certain company is into table tennis, strangely
- Popular copper entertains them periodically, beyond a doubt
- Certain tin, once damaged, fixed
- Fit after popular competition? It’s undeniable
- Certain popular trick experiment accomplished
- Fashionable artist – over time I’m fading away, beyond question
- Certain to be clean isn’t bad
- Clear obstacle in net at sea
- One new copper accepting hollow tale’s true
Buddy explains!

The straight part
We know that "Certain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case certain = incontestable.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 INCONTESTABLE mean?
INCONTESTABLE adjective- Not contestable; indisputable; certain