Built-in from dollar, get nickels back (8)
I believe the answer is:
integral
'built-in' is the definition.
(synonyms)
'from dollar get nickels back' is the wordplay.
'from' indicates the answer is hidden within the clue.
'back' says the letters should be written in reverse.
'largetni' is hidden in the letters of 'dollar get nickels'.
'largetni' back-to-front is 'INTEGRAL'.
(Other definitions for integral that I've seen before include "Altering so as to be necessary to the whole" , "Triangle/relating/altering/alerting (anag)" , "United" , "Unimpaired" , "Maths function; entire" .)