The most common and most likely answer to this crossword clue is the 10 letter word INEVITABLE
I've seen this crossword clue in The Daily Telegraph
I
N
E
V
I
T
A
B
L
E
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 = inevitable.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 INEVITABLE mean?
INEVITABLE noun- Something that is predictable, necessary, or cannot be avoided.
- Impossible to avoid or prevent.
- Predictable, or always happening.