Function that could be deadly? (3)

I believe the answer is:

sin

I'm a little stuck... Click here to teach me more about this clue!

'function' is the definition.
(mathematical function)

I cannot understand how the rest of the clue works.

(Other definitions for sin that I've seen before include "Mortal offence" , "Misdeed" , "'Transgression, mortal or venial (3)'" , "Depravity" , "Immoral action" .)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app