The definition is painting, referring to impasto, a technique of applying paint thickly. The wordplay is a charade: "This writer's" gives IM (a contraction of "I'm", where "this writer" = I), "quiet" gives P (piano in musical notation), "a" gives A from the surface, "holy person" gives ST (saint), and "old" gives O. Putting them together: IM + P + A + ST + O = IMPASTO.