The most common and most likely answer to this crossword clue is the 1 plus 7 plus 2 letter word I BELIEVE SO
I've seen this crossword clue in Independent
I
B
E
L
I
E
V
E
S
O
Buddy explains!
The straight part
We know that "That’s fairly sure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s fairly sure = i believe so.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.