The most common and most likely answer to this crossword clue is the 5 letter word SINES
I've seen this crossword clue in The Guardian
S
I
N
E
S
More clues leading to the result SINES
Buddy explains!

The straight part
We know that "functions" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case functions = sines.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 SINES mean?
SINES noun- In a right triangle, the ratio of the length of the side opposite an angle to the length of the hypotenuse.