The most common and most likely answer to this crossword clue is the 6 plus 3 plus 7 letter word DONNER AND BLITZEN
I've seen this crossword clue in The Sunday Telegraph
D
O
N
N
E
R
A
N
D
B
L
I
T
Z
E
N
Buddy explains!
The straight part
We know that "Tempestuous duo completing 16’s team of 31" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tempestuous duo completing 16’s team of 31 = donner and blitzen.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.