library(raster)

r <- raster()
set.seed(0)
values(r) <- runif(ncell(r), min = 0, max = 1000)
coords = cbind(runif(25, min = -100, max = 100),
               runif(25, min = -50, max = 50))

x <- rasterize(coords, r)
f <- focal(x, w=matrix(1, nc=3, nr=3), na.rm=TRUE)
rr <- mask(r, f)
slope <- terrain(rr, "slope")

extract(a, coords)
# [1] 0.0019366236 0.0020670699 0.0006305257 0.0025334280 0.0023480935 0.0007527267 0.0002699272 0.0004699626
# [9] 0.0004869054 0.0025651333 0.0010415805 0.0008574920 0.0010664869 0.0017700297 0.0001666226 0.0008405391
#[17] 0.0017682167 0.0009854172 0.0015350466 0.0017714466 0.0012994945 0.0016563132 0.0003276584 0.0020499529
#[25] 0.0006582073