If I understand correctly what you want, I think you should use a function like str_extract from the stringr package. Can you post a small example of the data you have and the result you want to get?
DF <- data.frame(TRG= "About half (41-60%) ()")
DF
TRG
1 About half (41-60%) ()
library(stringr)
DF <- DF %>% mutate(New = str_extract(TRG, "\\([^\\)]+"),
New = str_remove(New, "\\("),
TRG = str_extract(TRG,"^[^\\(]+"),
TRG = str_trim(TRG))
DF
TRG New
1 About half 41-60%