how to obtain only one cluster's data (.csv file) in Seurat
You can access data within the Seurat object using GetAssayData, and extract a list of cell names for the cluster you're interested in using WhichCells:

library(Seurat)

cells.use &lt;- WhichCells(object = pbmc_small, ident = 1)
expr &lt;- GetAssayData(object = pbmc_small, assay.type = "RNA", slot = "data")[, cells.use]
expr &lt;- as(Class = 'matrix', object = expr)
write.csv(x = expr, file = "expression_cluster1.csv", quote = FALSE)