The most common and most likely answer to this crossword clue is the 12 letter word DOUBLE-CHECK
I've seen this crossword clue in The Guardian
D
O
U
B
L
E
-
C
H
E
C
K
More clues leading to the result DOUBLE-CHECK
Buddy explains!
The straight part
We know that "Make certain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case make certain = double-check.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 DOUBLE-CHECK mean?
DOUBLE-CHECK verb- To check again (just to be sure)