Function hosted by Boris in error (4)
I believe the answer is:
sine
'function' is the definition.
(mathematical function)
'hosted by boris in error' is the wordplay.
'hosted by' indicates the answer is hidden within the clue (the answer is hosted or welcomed by the clue).
'SINE' is hidden in the letters of 'boris in error'.
(Other definitions for sine that I've seen before include "Trigonometric function" , "Without; maths function" , "Mathematical function" , "Trig function" , "Opposite/hypoteneuse" .)