The most common and most likely answer to this crossword clue is the 14 letter word UNQUESTIONABLE
I've seen this crossword clue in The Sunday Telegraph
U
N
Q
U
E
S
T
I
O
N
A
B
L
E
More clues leading to the result UNQUESTIONABLE
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 = unquestionable.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 UNQUESTIONABLE mean?
UNQUESTIONABLE adjective- Undeniable, obvious