Built-in (8)
I believe the answer is:
integral
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">built-in</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)