Built-in (8)
I believe the answer is:
integral
'built-in' is the definition.
(I know that built-in can be written as integral)
This is the entire clue.
(Other definitions for integral that I've seen before include "Necessary as part of a whole, like triangle" , "Maths function; whole" , "Built-in -- 1 across (anag)" , "Forming part of the whole" , "Triangle being essential" .)