Create new columns and change names

General
2

You can do something like this

library(tidyverse)

sample_df <- data.frame(
    stringsAsFactors = FALSE,
    original_column = c(
        "clone 9_+b_004.fcs/Single cells/red vs. green",
        "clone 9_+T_004.fcs/Single cells/red vs. green",
        "clone 10_+b_004.fcs/Single cells/red vs. green"
    )
)

sample_df %>% 
    separate(original_column,
             into = c("clone", "added_compound", "other_column"),
             sep = "_") %>% 
    mutate(added_compound = case_when(
        added_compound == "+b" ~ "TGFb",
        added_compound == "+T" ~ "Tamoxifen"
    ))
#>      clone added_compound                       other_column
#> 1  clone 9           TGFb 004.fcs/Single cells/red vs. green
#> 2  clone 9      Tamoxifen 004.fcs/Single cells/red vs. green
#> 3 clone 10           TGFb 004.fcs/Single cells/red vs. green

Created on 2020-03-08 by the reprex package (v0.3.0.9001)

If this doesn't solve your issue, please provide a proper REPRoducible EXample (reprex) illustrating your issue.