The most common and most likely answer to this crossword clue is the 1 plus 4 plus 3 plus 7 letter word A HEAD FOR FIGURES
I've seen this crossword clue in The Times
A
H
E
A
D
F
O
R
F
I
G
U
R
E
S
Buddy explains!
The straight part
We know that "what statistician needs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what statistician needs = a head for figures.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.